diff --git a/app/globals.css b/app/globals.css
index 3ed4e0f32..dfb768ca0 100644
--- a/app/globals.css
+++ b/app/globals.css
@@ -3,6 +3,23 @@
 
 @import "./theme-inline.css";
 
+/* The app's font stack replaces Tailwind's default --font-sans, so the
+   font-sans utility and body use the same stack, and a theme's
+   typography.fontSans (compiled to --font-sans on :root) restyles both. */
+@theme {
+  --font-sans:
+    -apple-system,
+    BlinkMacSystemFont,
+    "Segoe UI",
+    Roboto,
+    "Helvetica Neue",
+    Arial,
+    "Noto Sans Thai",
+    "Leelawadee UI",
+    Tahoma,
+    sans-serif;
+}
+
 :root {
   font-size: var(--font-size-base);
 
@@ -142,17 +159,7 @@
 body {
   background-color: var(--color-background);
   color: var(--color-foreground);
-  font-family:
-    -apple-system,
-    BlinkMacSystemFont,
-    "Segoe UI",
-    Roboto,
-    "Helvetica Neue",
-    Arial,
-    "Noto Sans Thai",
-    "Leelawadee UI",
-    Tahoma,
-    sans-serif;
+  font-family: var(--font-sans);
   font-feature-settings:
     "rlig" 1,
     "calt" 1;
diff --git a/lib/__tests__/font-sans.test.ts b/lib/__tests__/font-sans.test.ts
new file mode 100644
index 000000000..6a9ddfe63
--- /dev/null
+++ b/lib/__tests__/font-sans.test.ts
@@ -0,0 +1,54 @@
+import { describe, it, expect, beforeAll } from 'vitest';
+import { mkdtempSync, readFileSync, rmSync } from 'fs';
+import os from 'os';
+import path from 'path';
+import type { AtRule, Root } from 'postcss';
+
+/**
+ * A theme's typography.fontSans compiles to --font-sans on :root (#1278).
+ * Body has to read that variable, and the variable's default has to be the
+ * app's own stack, not Tailwind's, so nothing changes without a theme.
+ * Build globals.css with Tailwind to see the default it actually emits.
+ */
+let root: Root;
+
+beforeAll(async () => {
+  const postcss = (await import('postcss')).default;
+  const tailwind = (await import('@tailwindcss/postcss')).default;
+  const file = path.join(process.cwd(), 'app', 'globals.css');
+  // Scan an empty folder: only the stylesheet's own rules matter here.
+  const base = mkdtempSync(path.join(os.tmpdir(), 'font-sans-'));
+  try {
+    root = (await postcss([tailwind({ base })]).process(readFileSync(file, 'utf8'), { from: file })).root;
+  } finally {
+    rmSync(base, { recursive: true, force: true });
+  }
+}, 60_000);
+
+/**
+ * The value of `prop` in the `selector` rule whose parent is `wrapper`
+ * (`@layer theme`), or that sits at the top level when `wrapper` is null.
+ */
+function declValue(selector: string, prop: string, wrapper: string | null): string | undefined {
+  let value: string | undefined;
+  root.walkRules(selector, (rule) => {
+    const at = rule.parent?.type === 'atrule' ? (rule.parent as AtRule) : null;
+    if (value !== undefined || (at ? `@${at.name} ${at.params}` : null) !== wrapper) return;
+    rule.walkDecls(prop, (decl) => {
+      value ??= decl.value.replace(/\s+/g, ' ');
+    });
+  });
+  return value;
+}
+
+describe('theme font (#1278)', () => {
+  it('sets body in --font-sans', () => {
+    expect(declValue('body', 'font-family', null)).toBe('var(--font-sans)');
+  });
+
+  it('defaults --font-sans to the app stack, Thai fallbacks included', () => {
+    expect(declValue(':root, :host', '--font-sans', '@layer theme')).toBe(
+      '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans Thai", "Leelawadee UI", Tahoma, sans-serif',
+    );
+  });
+});