diff --git a/plugins/README.md b/plugins/README.md
index c9d97c5f..4765e421 100644
--- a/plugins/README.md
+++ b/plugins/README.md
@@ -384,6 +384,23 @@ inline scripts and styles and `data:` images, no network, no IDE API — and
 talks to the plugin only through `openidea.postMessage(text)` /
 `openidea.onMessage(fn)` (`ViewEvent::Message`, `host::post_view_message`).
 
+The frame follows the IDE theme without reloading: its root carries
+`data-theme="light|dark"` and these CSS variables, with concrete colour
+values (the body already uses the text colour, UI font and size):
+
+| Variable                                                                                                          | Meaning                                                          |
+| ----------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------- |
+| `--openidea-bg`, `--openidea-bg-elevated`, `--openidea-bg-hover`, `--openidea-bg-selected`, `--openidea-bg-input` | Surfaces (the frame itself is transparent)                       |
+| `--openidea-fg`, `--openidea-fg-strong`, `--openidea-fg-muted`                                                    | Text                                                             |
+| `--openidea-border`, `--openidea-focus-ring`, `--openidea-link`                                                   | Lines, focus, links                                              |
+| `--openidea-accent`, `--openidea-fg-on-accent`                                                                    | Accent and text on it                                            |
+| `--openidea-danger`, `--openidea-warning`, `--openidea-success`, `--openidea-info`                                | Status colours                                                   |
+| `--openidea-font-ui`, `--openidea-font-mono`, `--openidea-font-size`                                              | Fonts (web fonts the IDE bundles are not available in the frame) |
+
+Scripts get the same as `openidea.theme` (`{ dark, vars }`) and
+`openidea.onThemeChange(fn)`, for drawing on a canvas, say. Variables are only
+ever added.
+
 ### Settings
 
 ```toml
diff --git a/src/features/plugins/__tests__/views.test.tsx b/src/features/plugins/__tests__/views.test.tsx
index a78fce47..8d197d15 100644
--- a/src/features/plugins/__tests__/views.test.tsx
+++ b/src/features/plugins/__tests__/views.test.tsx
@@ -9,7 +9,7 @@ import { defineBuiltinUiPoints } from "../points";
 import { clearUiContributions, syncUiContributions } from "../projection";
 import { pluginMenuItems, withPluginSetting } from "../uiPoints";
 import { matchesPattern } from "../views/customEditor";
-import { FRAME_CSP, frameDocument } from "../views/frame";
+import { FRAME_CSP, FRAME_TAG, frameDocument, frameTheme, type FrameTheme } from "../views/frame";
 import { PluginView, treeNodes, wantedViewport } from "../views/PluginView";
 import { bumpView } from "../views/viewStore";
 
@@ -56,7 +56,7 @@ describe("view helpers", () => {
   });
 
   it("confines HTML bodies: CSP without network, a message bridge", () => {
-    const doc = frameDocument("<p>hi</p>", true);
+    const doc = frameDocument("<p>hi</p>", { dark: true, vars: {} });
     expect(doc).toContain(`content="${FRAME_CSP}"`);
     expect(FRAME_CSP).toContain("default-src 'none'");
     expect(FRAME_CSP).not.toMatch(/https?:|connect-src/);
@@ -64,6 +64,71 @@ describe("view helpers", () => {
     expect(doc.indexOf("Content-Security-Policy")).toBeLessThan(doc.indexOf("<p>hi</p>"));
   });
 
+  it("reads the frame theme from the IDE tokens, resolved and sanitised", () => {
+    const root = document.documentElement;
+    const before = root.dataset.theme;
+    root.style.setProperty("--fg", "#112233");
+    root.style.setProperty("--font-ui", "Inter, sans-serif");
+    root.style.setProperty("--font-mono", "x}</style><script>alert(1)</script>");
+    try {
+      root.dataset.theme = "light";
+      const light = frameTheme();
+      expect(light.dark).toBe(false);
+      expect(light.vars["--openidea-fg"]).toBe("rgb(17, 34, 51)");
+      expect(light.vars["--openidea-font-ui"]).toBe("Inter, sans-serif");
+      expect(light.vars).not.toHaveProperty("--openidea-font-mono");
+      root.dataset.theme = "dark";
+      root.dataset.contrast = "high";
+      expect(frameTheme().dark).toBe(true);
+    } finally {
+      for (const p of ["--fg", "--font-ui", "--font-mono"]) root.style.removeProperty(p);
+      delete root.dataset.contrast;
+      if (before === undefined) delete root.dataset.theme;
+      else root.dataset.theme = before;
+    }
+  });
+
+  it("puts the theme into the frame document without letting a value end the script", () => {
+    const doc = frameDocument("<p>hi</p>", { dark: true, vars: { "--openidea-fg": "</script><b>" } });
+    expect(doc).toContain('<html data-theme="dark">');
+    expect(doc).not.toContain("</script><b>");
+    expect(doc).toContain("\\u003c/script>");
+  });
+
+  it("applies posted themes inside the frame, replacing the previous variables", () => {
+    const doc = frameDocument("", {
+      dark: false,
+      vars: { "--openidea-fg": "rgb(1, 2, 3)", "--openidea-accent": "red" },
+    });
+    const script = /<script>([\s\S]*)<\/script>/.exec(doc)![1]!;
+    const root = document.documentElement;
+    const before = root.dataset.theme;
+    const seen: unknown[] = [];
+    try {
+      new Function(script)();
+      const api = (
+        window as unknown as { openidea: { theme: FrameTheme; onThemeChange(fn: (t: FrameTheme) => void): void } }
+      ).openidea;
+      expect(root.style.getPropertyValue("--openidea-fg")).toBe("rgb(1, 2, 3)");
+      expect(api.theme.dark).toBe(false);
+      api.onThemeChange((t) => seen.push(t));
+      // jsdom's postMessage has no `source`, so dispatch the event as the parent would.
+      const send = (t: unknown) =>
+        window.dispatchEvent(new MessageEvent("message", { data: { tag: FRAME_TAG, theme: t }, source: window }));
+      send({ dark: true, vars: { "--openidea-fg": "rgb(9, 9, 9)", "--not-ours": "x" } });
+      expect(root.dataset.theme).toBe("dark");
+      expect(root.style.getPropertyValue("--openidea-fg")).toBe("rgb(9, 9, 9)");
+      expect(root.style.getPropertyValue("--openidea-accent")).toBe("");
+      expect(root.style.getPropertyValue("--not-ours")).toBe("");
+      expect(seen).toHaveLength(1);
+    } finally {
+      for (const p of ["--openidea-fg", "--openidea-accent"]) root.style.removeProperty(p);
+      delete (window as unknown as { openidea?: unknown }).openidea;
+      if (before === undefined) delete root.dataset.theme;
+      else root.dataset.theme = before;
+    }
+  });
+
   it("matches custom editor file name patterns", () => {
     expect(matchesPattern("*.csv", "People.CSV")).toBe(true);
     expect(matchesPattern("*.{csv,tsv}", "a.tsv")).toBe(true);
@@ -185,6 +250,30 @@ describe("PluginView", () => {
     expect(await screen.findByText("Paris")).toBeInTheDocument();
   });
 
+  it("keeps an HTML body loaded across theme switches and posts the new theme to it", async () => {
+    const html: View = { toolbar: [], body: { kind: "html", value: "<p>chart</p>" }, status: null };
+    setPluginsBackend(backendWith(() => html, []));
+    const root = document.documentElement;
+    const before = root.dataset.theme;
+    root.dataset.theme = "light";
+    try {
+      const { container } = render(<PluginView pluginId="charts" view="chart" />);
+      await waitFor(() => expect(container.querySelector("iframe")).not.toBeNull());
+      const frame = container.querySelector("iframe")!;
+      const srcDoc = frame.getAttribute("srcdoc");
+      expect(srcDoc).toContain('<html data-theme="light">');
+      const post = vi.spyOn(frame.contentWindow!, "postMessage");
+      root.dataset.theme = "dark";
+      await waitFor(() =>
+        expect(post).toHaveBeenCalledWith({ tag: FRAME_TAG, theme: expect.objectContaining({ dark: true }) }, "*"),
+      );
+      expect(frame.getAttribute("srcdoc")).toBe(srcDoc);
+    } finally {
+      if (before === undefined) delete root.dataset.theme;
+      else root.dataset.theme = before;
+    }
+  });
+
   it("shows a message when the plugin fails", async () => {
     const base = createMemoryPluginsBackend([]);
     setPluginsBackend({ ...base, renderView: () => Promise.reject(new Error("boom")) });
diff --git a/src/features/plugins/views/PluginView.tsx b/src/features/plugins/views/PluginView.tsx
index 343c5005..33406140 100644
--- a/src/features/plugins/views/PluginView.tsx
+++ b/src/features/plugins/views/PluginView.tsx
@@ -6,7 +6,15 @@
  * (`viewChanged`), from `revision` (a custom editor's document changed), or
  * from the table scrolling to rows it hasn't got.
  */
-import { useCallback, useEffect, useMemo, useRef, useState, type KeyboardEvent as ReactKeyboardEvent } from "react";
+import {
+  useCallback,
+  useEffect,
+  useMemo,
+  useRef,
+  useState,
+  useSyncExternalStore,
+  type KeyboardEvent as ReactKeyboardEvent,
+} from "react";
 import type { View, ViewEvent, ViewField, ViewTable, ViewTreeItem, Viewport } from "@/bindings";
 import { tx } from "@/platform/i18n";
 import {
@@ -22,7 +30,8 @@ import {
   type MenuItem,
 } from "@/ui";
 import { pluginsBackend } from "../backend";
-import { FRAME_TAG, frameDocument } from "./frame";
+import { subscribeTheme, themeKey } from "@/platform/themeWatch";
+import { FRAME_TAG, frameDocument, frameTheme, frameThemeMessage } from "./frame";
 import { onViewMessage, useViewRevision } from "./viewStore";
 import styles from "./PluginView.module.css";
 
@@ -456,8 +465,15 @@ function HtmlBody({
   view: string;
 }) {
   const frame = useRef<HTMLIFrameElement>(null);
-  const dark = typeof document !== "undefined" && document.documentElement.dataset.theme === "dark";
-  const srcDoc = useMemo(() => frameDocument(html, dark), [html, dark]);
+  // The theme at load time is in the document; later changes are posted, so a
+  // theme switch doesn't reload the frame and lose its state.
+  const srcDoc = useMemo(() => frameDocument(html, frameTheme()), [html]);
+  const theme = useSyncExternalStore(subscribeTheme, themeKey, () => "");
+  const postTheme = useCallback(
+    () => frame.current?.contentWindow?.postMessage(frameThemeMessage(frameTheme()), "*"),
+    [],
+  );
+  useEffect(postTheme, [theme, postTheme]);
 
   useEffect(() => {
     const onMessage = (e: MessageEvent) => {
@@ -476,5 +492,15 @@ function HtmlBody({
   }, [pluginId, view, send]);
 
   // No `allow-same-origin`: an opaque origin without access to the IDE.
-  return <iframe ref={frame} className={styles.frame} title={view} sandbox="allow-scripts" srcDoc={srcDoc} />;
+  // `onLoad` re-sends the theme in case it changed between building `srcDoc` and the frame loading.
+  return (
+    <iframe
+      ref={frame}
+      className={styles.frame}
+      title={view}
+      sandbox="allow-scripts"
+      srcDoc={srcDoc}
+      onLoad={postTheme}
+    />
+  );
 }
diff --git a/src/features/plugins/views/frame.ts b/src/features/plugins/views/frame.ts
index 8ea26b98..7f4a1255 100644
--- a/src/features/plugins/views/frame.ts
+++ b/src/features/plugins/views/frame.ts
@@ -4,7 +4,13 @@
  * access) under a CSP that allows only its own inline scripts and styles and
  * `data:` images — no network. `openidea.postMessage` / `openidea.onMessage`
  * are its only way to the plugin.
+ *
+ * The IDE theme reaches the frame as `data-theme="light|dark"` on its root and
+ * the `--openidea-*` variables of {@link FRAME_THEME_TOKENS}, kept current
+ * without reloading the frame; `openidea.theme` / `openidea.onThemeChange` give
+ * the same to scripts (canvas drawing, say).
  */
+import { resolveTokenColor, toRgb } from "@/platform/cssColor";
 
 export const FRAME_CSP =
   "default-src 'none'; script-src 'unsafe-inline'; style-src 'unsafe-inline'; img-src data:; font-src data:; media-src data:";
@@ -12,27 +18,110 @@ export const FRAME_CSP =
 /** Marks frame ↔ workbench messages. */
 export const FRAME_TAG = "openidea-view";
 
-const BRIDGE = `
+/**
+ * The theme variables a frame gets: public name → IDE token. Part of the
+ * plugin API: names are only ever added, so the IDE's own tokens can change.
+ */
+export const FRAME_THEME_TOKENS = {
+  "--openidea-bg": "--bg-island",
+  "--openidea-bg-elevated": "--bg-elev",
+  "--openidea-bg-hover": "--bg-hover",
+  "--openidea-bg-selected": "--bg-tree-selected",
+  "--openidea-bg-input": "--bg-input",
+  "--openidea-fg": "--fg",
+  "--openidea-fg-strong": "--fg-strong",
+  "--openidea-fg-muted": "--fg-muted",
+  "--openidea-border": "--border",
+  "--openidea-accent": "--accent",
+  "--openidea-fg-on-accent": "--fg-on-accent",
+  "--openidea-link": "--link",
+  "--openidea-focus-ring": "--focus-ring",
+  "--openidea-danger": "--danger",
+  "--openidea-warning": "--warning",
+  "--openidea-success": "--success",
+  "--openidea-info": "--info",
+} as const;
+
+/** Non-colour variables: public name → IDE token. */
+const FRAME_FONT_TOKENS = {
+  "--openidea-font-ui": "--font-ui",
+  "--openidea-font-mono": "--font-mono",
+  "--openidea-font-size": "--fs-ui",
+} as const;
+
+export interface FrameTheme {
+  dark: boolean;
+  /** `--openidea-*` → a concrete value (colours resolved, no `var()`: the frame has no IDE stylesheet). */
+  vars: Record<string, string>;
+}
+
+/** Font stacks and sizes only; anything that could leave its declaration is dropped. */
+const safeValue = (v: string) => v.length <= 400 && !/[<>{};\\]/.test(v);
+
+/** The IDE theme as a frame sees it, read from the root element's tokens. */
+export function frameTheme(root: HTMLElement = document.documentElement): FrameTheme {
+  const vars: Record<string, string> = {};
+  for (const [name, token] of Object.entries(FRAME_THEME_TOKENS)) {
+    const c = resolveTokenColor(token, root);
+    if (c) vars[name] = toRgb(c);
+  }
+  if (typeof getComputedStyle !== "undefined") {
+    const css = getComputedStyle(root);
+    for (const [name, token] of Object.entries(FRAME_FONT_TOKENS)) {
+      const v = css.getPropertyValue(token).trim();
+      if (v && safeValue(v)) vars[name] = v;
+    }
+  }
+  return { dark: root.dataset.theme !== "light", vars };
+}
+
+/** The message that updates a loaded frame's theme. */
+export const frameThemeMessage = (theme: FrameTheme) => ({ tag: FRAME_TAG, theme });
+
+const bridge = (initial: FrameTheme) => `
 (() => {
   const listeners = [];
+  const themeListeners = [];
+  const root = document.documentElement;
+  let theme = null;
+  let applied = [];
+  const applyTheme = (t) => {
+    if (!t || typeof t !== "object" || !t.vars || typeof t.vars !== "object") return;
+    const vars = {};
+    for (const [k, v] of Object.entries(t.vars)) if (k.startsWith("--openidea-") && typeof v === "string") vars[k] = v;
+    for (const k of applied) if (!(k in vars)) root.style.removeProperty(k);
+    for (const [k, v] of Object.entries(vars)) root.style.setProperty(k, v);
+    applied = Object.keys(vars);
+    root.dataset.theme = t.dark ? "dark" : "light";
+    theme = Object.freeze({ dark: !!t.dark, vars: Object.freeze(vars) });
+    for (const l of themeListeners) { try { l(theme); } catch (err) { console.error(err); } }
+  };
+  applyTheme(${JSON.stringify(initial).replace(/</g, "\\u003c")});
   window.addEventListener("message", (e) => {
     if (e.source !== window.parent || !e.data || e.data.tag !== "${FRAME_TAG}") return;
+    if (e.data.theme) { applyTheme(e.data.theme); return; }
     for (const l of listeners) { try { l(String(e.data.text)); } catch (err) { console.error(err); } }
   });
   window.openidea = Object.freeze({
     postMessage: (text) => window.parent.postMessage({ tag: "${FRAME_TAG}", text: String(text) }, "*"),
     onMessage: (fn) => { if (typeof fn === "function") listeners.push(fn); },
+    get theme() { return theme; },
+    onThemeChange: (fn) => { if (typeof fn === "function") themeListeners.push(fn); },
   });
 })();`;
 
-/** `html` wrapped with the CSP, the message bridge and base styles. */
-export function frameDocument(html: string, dark: boolean): string {
-  const scheme = dark ? "dark" : "light";
+const BASE_STYLE =
+  "html[data-theme=light]{color-scheme:light}html[data-theme=dark]{color-scheme:dark}" +
+  "body{margin:8px;color:var(--openidea-fg);font-family:var(--openidea-font-ui,system-ui,sans-serif);" +
+  "font-size:var(--openidea-font-size,13px)}a{color:var(--openidea-link)}";
+
+/** `html` wrapped with the CSP, the message bridge, the theme and base styles. */
+export function frameDocument(html: string, theme: FrameTheme): string {
   return [
-    '<!doctype html><html><head><meta charset="utf-8">',
+    `<!doctype html><html data-theme="${theme.dark ? "dark" : "light"}"><head><meta charset="utf-8">`,
     `<meta http-equiv="Content-Security-Policy" content="${FRAME_CSP}">`,
-    `<style>:root{color-scheme:${scheme}}body{margin:8px;font:13px system-ui,sans-serif}</style>`,
-    `<script>${BRIDGE}</script>`,
+    `<style>${BASE_STYLE}</style>`,
+    `<script>${bridge(theme)}</script>`,
     "</head><body>",
     html,
     "</body></html>",
diff --git a/src/features/viewers/markdown/MarkdownEditor.tsx b/src/features/viewers/markdown/MarkdownEditor.tsx
index 18b0895a..56531525 100644
--- a/src/features/viewers/markdown/MarkdownEditor.tsx
+++ b/src/features/viewers/markdown/MarkdownEditor.tsx
@@ -25,7 +25,8 @@ import { setShownLine } from "@/editor/api";
 import { highlightInto } from "@/editor/lsp/markdown";
 import { FloatingToolbar, selectableText } from "@/ui";
 import { publishPreviewTop, useLiveMarkdown } from "./live";
-import { hasMermaid, MERMAID_SELECTOR, renderMermaidIn, subscribeTheme, themeKey } from "./mermaid";
+import { subscribeTheme, themeKey } from "@/platform/themeWatch";
+import { hasMermaid, MERMAID_SELECTOR, renderMermaidIn } from "./mermaid";
 import { modeOf, useMarkdownModes } from "./modes";
 import { MarkdownLayoutActions } from "./MarkdownLayoutActions";
 import {
diff --git a/src/features/viewers/markdown/mermaid.test.ts b/src/features/viewers/markdown/mermaid.test.ts
index fe63bbb3..80b8b8bb 100644
--- a/src/features/viewers/markdown/mermaid.test.ts
+++ b/src/features/viewers/markdown/mermaid.test.ts
@@ -1,6 +1,7 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import { renderMarkdown } from "./render";
-import { hasMermaid, renderMermaidIn, resetMermaidForTests, subscribeTheme, themeKey } from "./mermaid";
+import { subscribeTheme, themeKey } from "@/platform/themeWatch";
+import { hasMermaid, renderMermaidIn, resetMermaidForTests } from "./mermaid";
 
 const m = vi.hoisted(() => ({
   loads: 0,
diff --git a/src/features/viewers/markdown/mermaid.ts b/src/features/viewers/markdown/mermaid.ts
index cce3cca2..fbd8bd0d 100644
--- a/src/features/viewers/markdown/mermaid.ts
+++ b/src/features/viewers/markdown/mermaid.ts
@@ -6,6 +6,7 @@
 import { fontsLoaded } from "@/platform/fonts";
 import type { Mermaid } from "mermaid";
 import { resolveTokenColor, toRgb } from "@/platform/cssColor";
+import { themeKey } from "@/platform/themeWatch";
 import { attachPanZoom } from "../panZoom";
 
 export const MERMAID_SELECTOR = "[data-mermaid]";
@@ -29,26 +30,6 @@ function loadMermaid(): Promise<Mermaid> {
   return loading;
 }
 
-/** Theme identity: anything that changes the colours read below. */
-export function themeKey(): string {
-  const root = document.documentElement;
-  const { theme = "", contrast = "", colorTheme = "", pluginTheme = "" } = root.dataset;
-  const accent = root.style.getPropertyValue("--accent");
-  const font = getComputedStyle(root).getPropertyValue("--font-ui");
-  return `${theme}|${contrast}|${colorTheme}|${pluginTheme}|${accent}|${font}`;
-}
-
-/** Calls `cb` when the theme attributes (a plugin theme's included) or the accent on the root change. */
-export function subscribeTheme(cb: () => void): () => void {
-  if (typeof MutationObserver === "undefined") return () => {};
-  const observer = new MutationObserver(cb);
-  observer.observe(document.documentElement, {
-    attributes: true,
-    attributeFilter: ["data-theme", "data-contrast", "data-color-theme", "data-plugin-theme", "style"],
-  });
-  return () => observer.disconnect();
-}
-
 /**
  * Diagram colours from the IDE tokens. Mermaid parses only hex and `rgb()`, so
  * tokens a theme writes as `hsl()`, `color-mix()` or `var()` are resolved to RGB.
diff --git a/src/platform/themeWatch.ts b/src/platform/themeWatch.ts
new file mode 100644
index 00000000..7cdfe530
--- /dev/null
+++ b/src/platform/themeWatch.ts
@@ -0,0 +1,25 @@
+/**
+ * Watching the active IDE theme: built-in theme, palette theme, plugin theme
+ * and accent all live as attributes / inline style on the root element.
+ */
+
+/** Theme identity: changes whenever the colours the theme tokens resolve to may change. */
+export function themeKey(): string {
+  const root = document.documentElement;
+  const { theme = "", contrast = "", colorTheme = "", pluginTheme = "" } = root.dataset;
+  const accent = root.style.getPropertyValue("--accent");
+  // The UI font (the custom one, or its fallback once loaded) is part of how a theme renders.
+  const font = getComputedStyle(root).getPropertyValue("--font-ui");
+  return `${theme}|${contrast}|${colorTheme}|${pluginTheme}|${accent}|${font}`;
+}
+
+/** Calls `cb` when the theme attributes (a plugin theme's included) or the accent on the root change. */
+export function subscribeTheme(cb: () => void): () => void {
+  if (typeof MutationObserver === "undefined") return () => {};
+  const observer = new MutationObserver(cb);
+  observer.observe(document.documentElement, {
+    attributes: true,
+    attributeFilter: ["data-theme", "data-contrast", "data-color-theme", "data-plugin-theme", "style"],
+  });
+  return () => observer.disconnect();
+}