fix: show plain-text mail in the dark theme's own colours instead of a fixed black

src/components/EmailBodyView.tsx

@@ -688,6 +688,13 @@ export default function EmailBodyView({
   // Sanitised and wrapped once per message, body and settings, and reused
   // when the same body mounts again (lib/email-body-document).
   const emptyLabel = t('email_viewer.no_body_content', '(No body content available)');
+  // A plain-text body in a dark app theme takes the theme's colours, not a
+  // fixed #0a0a0a. Only while the app itself is dark: a single message
+  // switched to dark in a light app has no dark palette to borrow.
+  const plainDarkColors = React.useMemo(
+    () => (resolvedTheme === 'dark' ? { background: c.background, text: c.text, link: c.textLink } : undefined),
+    [resolvedTheme, c.background, c.text, c.textLink],
+  );
   const prepared = React.useMemo(() => bodyDocument({
     key: `${jmapAccountId ?? ''}|${email.id}`,
     rawHtml,
@@ -700,9 +707,10 @@ export default function EmailBodyView({
     plainTextFont,
     quoteLabels,
     textFactor,
+    plainDarkColors,
   }), [
     jmapAccountId, email.id, rawHtml, text, email.preview, emptyLabel, shouldBlock, cidMap, renderAsDark,
-    messageSpacing, plainTextFont, quoteLabels, textFactor,
+    messageSpacing, plainTextFont, quoteLabels, textFactor, plainDarkColors,
   ]);
 
   const source = React.useMemo(() => ({ html: prepared.html }), [prepared.html]);

src/lib/__tests__/plain-text-theme-colors.test.ts

@@ -0,0 +1,43 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+import { buildBodyDocument } from '../email-body-document';
+import { BUILTIN_THEMES } from '../../theme/builtin-themes';
+
+// A plain-text mail read in dark mode sat on a fixed #0a0a0a whatever the
+// app's theme was (UX audit 2026-10, "Plain-text mail dark background").
+
+const base = {
+  key: 'k', emptyLabel: '-', blockRemoteImages: false, cidMap: {}, rawHtml: null, text: 'hello',
+  messageSpacing: 'auto' as const, plainTextFont: 'sans' as const, quoteLabels: { show: 's', hide: 'h' },
+};
+
+describe('plain-text mail colours', () => {
+  it("takes the dark theme's ground, text and link colours", () => {
+    const nord = { background: '#2e3440', text: '#eceff4', link: '#88c0d0' };
+    const html = buildBodyDocument({ ...base, isDark: true, plainDarkColors: nord }).html;
+    expect(html).toContain('background: #2e3440;');
+    expect(html).toContain('color: #eceff4;');
+    expect(html).toContain('a { color: #88c0d0;');
+    expect(html).not.toContain('#0a0a0a');
+  });
+
+  it('keeps white in light mode and the old dark colours without a theme', () => {
+    const colors = { background: '#2e3440', text: '#eceff4', link: '#88c0d0' };
+    expect(buildBodyDocument({ ...base, isDark: false, plainDarkColors: colors }).html).toContain('background: #ffffff;');
+    expect(buildBodyDocument({ ...base, isDark: true }).html).toContain('background: #0a0a0a;');
+  });
+
+  it('cannot close the style sheet with a theme colour', () => {
+    const html = buildBodyDocument({
+      ...base, isDark: true, plainDarkColors: { background: 'red;}</style><script>x()</script>', text: '#fff', link: '#00f' },
+    }).html;
+    expect(html).not.toContain('</style><script>');
+  });
+
+  it('is fed from the app theme by the body view', () => {
+    expect(BUILTIN_THEMES.length).toBeGreaterThan(0);
+    const view = fs.readFileSync(path.resolve(__dirname, '../../components/EmailBodyView.tsx'), 'utf8');
+    expect(view).toMatch(/background: c\.background, text: c\.text, link: c\.textLink/);
+  });
+});

src/lib/email-body-document.ts

@@ -6,7 +6,7 @@
 
 import {
   prepareEmailHtml, wrapPlainTextEmail, plainTextToSafeHtml,
-  type MessageSpacing, type PlainTextFont,
+  type MessageSpacing, type PlainTextFont, type PlainTextColors,
 } from './email-html';
 import { collapsePlainTextQuotes, type QuoteCollapseLabels } from './quote-collapse';
 import { mailBaseFontPx } from './mail-reading';
@@ -31,6 +31,8 @@ export interface BodyDocumentInput {
    * body. An HTML body is scaled by an injected pass instead. Default 1.
    */
   textFactor?: number;
+  /** The app theme's colours for a plain-text body in dark mode. */
+  plainDarkColors?: PlainTextColors;
 }
 
 export interface BodyDocument {
@@ -61,7 +63,12 @@ export function buildBodyDocument(input: BodyDocumentInput): BodyDocument {
   }
   const safe = collapsePlainTextQuotes(plainTextToSafeHtml(text), input.quoteLabels);
   return {
-    html: wrapPlainTextEmail(safe, { isDark, font: input.plainTextFont, fontPx: mailBaseFontPx(input.textFactor ?? 1) }),
+    html: wrapPlainTextEmail(safe, {
+      isDark,
+      font: input.plainTextFont,
+      fontPx: mailBaseFontPx(input.textFactor ?? 1),
+      darkColors: input.plainDarkColors,
+    }),
     applyInversion: false,
     hasNativeDark: false,
     blockedExternal: false,
@@ -90,6 +97,7 @@ export function bodyDocument(input: BodyDocumentInput): BodyDocument {
   const key = JSON.stringify([
     input.key, input.blockRemoteImages, input.isDark, input.messageSpacing, input.plainTextFont,
     input.quoteLabels.show, input.quoteLabels.hide, input.emptyLabel, input.textFactor ?? 1,
+    input.plainDarkColors ?? null,
   ]);
   const hit = documents.get(key);
   if (hit && sameSources(hit.input, input)) {

src/lib/email-html.ts

@@ -953,16 +953,32 @@ export type PlainTextFont = 'sans' | 'mono';
 // WebView. Mirrors the webmail's plain-text render path: app font by default
 // (monospace on request, #830), `white-space: pre-wrap`, native dark theme
 // (no filter inversion), and a dark-mode-appropriate link colour.
+/** The app theme's colours for a plain-text body read in dark mode. */
+export interface PlainTextColors {
+  background: string;
+  text: string;
+  link: string;
+}
+
+/** A theme colour for a style sheet: nothing that could end the declaration or the element. */
+function cssColor(value: string, fallback: string): string {
+  const cleaned = value.replace(/[<>{};\\]/g, '').trim();
+  return cleaned || fallback;
+}
+
 export function wrapPlainTextEmail(
   innerHtml: string,
-  options: { isDark?: boolean; font?: PlainTextFont; fontPx?: number } = {},
+  options: { isDark?: boolean; font?: PlainTextFont; fontPx?: number; darkColors?: PlainTextColors } = {},
 ): string {
-  const { isDark = true, font = 'sans', fontPx = 14 } = options;
+  const { isDark = true, font = 'sans', fontPx = 14, darkColors } = options;
   const cleaned = stripDangerousTags(innerHtml);
 
-  const bg = isDark ? '#0a0a0a' : '#ffffff';
+  // In dark mode the body takes the app theme's ground, text and link colours
-  const fg = isDark ? '#fafafa' : '#1a1a1a';
+  // when they are given (a built-in dark theme isn't #0a0a0a); light mode
-  const link = isDark ? '#60a5fa' : '#1a73e8';
+  // stays on white like HTML mail.
+  const bg = isDark ? cssColor(darkColors?.background ?? '', '#0a0a0a') : '#ffffff';
+  const fg = isDark ? cssColor(darkColors?.text ?? '', '#fafafa') : '#1a1a1a';
+  const link = isDark ? cssColor(darkColors?.link ?? '', '#60a5fa') : '#1a73e8';
   const fontFamily = font === 'mono'
     ? 'ui-monospace, Menlo, Consolas, "SF Mono", monospace'
     : '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif';