diff --git a/frontend/package.json b/frontend/package.json
index 11b6fd5..c734d12 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -10,7 +10,8 @@
     "preview": "vite preview",
     "smoke": "node smoke.test.mjs",
     "test:token": "node token.test.mjs",
-    "verify": "vite build && node smoke.test.mjs && node token.test.mjs"
+    "test:theme": "node theme.test.mjs",
+    "verify": "vite build && node smoke.test.mjs && node token.test.mjs && node theme.test.mjs"
   },
   "dependencies": {
     "@tanstack/react-query": "^5.101.4",
diff --git a/frontend/src/theme/ThemeProvider.jsx b/frontend/src/theme/ThemeProvider.jsx
index fa9a61d..dc785fe 100644
--- a/frontend/src/theme/ThemeProvider.jsx
+++ b/frontend/src/theme/ThemeProvider.jsx
@@ -47,6 +47,35 @@ export function useTheme() {
   return ctx
 }
 
+/**
+ * A counter that increments every time [data-theme] flips. Use it as an effect
+ * or useMemo dependency.
+ *
+ * Almost nothing needs this: a CSS custom property change repaints the whole
+ * document for free, which is why the app re-themes without any React
+ * involvement. It exists for the handful of places that CANNOT ride a variable
+ * — anything that bakes a colour into a canvas, a string, or a separate
+ * document at render time. Those read the palette through getComputedStyle
+ * exactly once and then hold a stale copy forever, because changing a custom
+ * property repaints CSS but never re-runs JavaScript.
+ *
+ * It watches the ATTRIBUTE rather than subscribing to this provider's state, on
+ * purpose. initTheme() runs before React mounts and applyTheme() can be called
+ * from outside the tree, so the attribute is the only source that is always
+ * current — and a component using this hook then needs no provider at all.
+ * Chart.jsx observes the same attribute directly, for the same reason.
+ */
+export function useThemeVersion() {
+  const [version, setVersion] = useState(0)
+  useEffect(() => {
+    if (typeof MutationObserver !== 'function') return undefined
+    const mo = new MutationObserver(() => setVersion((v) => v + 1))
+    mo.observe(document.documentElement, { attributes: true, attributeFilter: ['data-theme'] })
+    return () => mo.disconnect()
+  }, [])
+  return version
+}
+
 export default function ThemeProvider({ children }) {
   const [theme, setThemeState] = useState(
     () => document.documentElement.getAttribute('data-theme') || 'light',
diff --git a/frontend/src/ui/EmailBody.jsx b/frontend/src/ui/EmailBody.jsx
index 2c80ee3..443f400 100644
--- a/frontend/src/ui/EmailBody.jsx
+++ b/frontend/src/ui/EmailBody.jsx
@@ -21,9 +21,16 @@
    Remote images stay blocked until the user asks for them. A tracking pixel in
    an applicant email would otherwise tell the sender exactly when a recruiter
    opened it.
+
+   That CSS isolation has one cost worth stating plainly: custom properties do
+   not inherit across an iframe boundary, so this is the only component in the
+   app that cannot re-theme itself for free. Its palette is snapshotted into the
+   frame's <style> at build time, and useThemeVersion is what reruns that build.
    ============================================================ */
 
-import { useCallback, useEffect, useRef, useState } from 'react'
+import { useCallback, useEffect, useMemo, useRef, useState } from 'react'
+
+import { useThemeVersion } from '../theme/ThemeProvider'
 
 /** Elements that have no business in a rendered email. */
 const STRIP_TAGS = [
@@ -64,12 +71,20 @@ function sanitize(html) {
   return doc.body?.innerHTML || ''
 }
 
-/** Inherit the host theme's colours so the email does not glare in dark mode. */
+/**
+ * Inherit the host theme's colours so the email does not glare in dark mode.
+ *
+ * Called fresh on every srcDoc build — see the themeVersion dependency below.
+ * Read the attribute for color-scheme rather than sniffing whether --bg happens
+ * to start with an "f": that guess quietly inverts the frame's form controls and
+ * scrollbars the moment a palette changes shade.
+ */
 function frameStyles() {
   const css = getComputedStyle(document.documentElement)
   const pick = (name, fallback) => (css.getPropertyValue(name) || fallback).trim()
+  const dark = document.documentElement.getAttribute('data-theme') === 'dark'
   return `
-    :root { color-scheme: ${pick('--bg', '#fff').startsWith('#f') ? 'light' : 'dark'}; }
+    :root { color-scheme: ${dark ? 'dark' : 'light'}; }
     body {
       margin: 0;
       background: ${pick('--bg-sunken', '#f7f7f8')};
@@ -113,8 +128,21 @@ export default function EmailBody({ html, maxHeight }) {
   const [allowRemoteImages, setAllowRemoteImages] = useState(false)
   const [height, setHeight] = useState(320)
   const [blockedImages, setBlockedImages] = useState(0)
+  const themeVersion = useThemeVersion()
 
-  const clean = sanitize(html)
+  // Both memoised because srcDoc IS the frame's source: handing React a new but
+  // equal string tears the document down and rebuilds it. Unmemoised, every
+  // unrelated parent render re-parsed the mail and reloaded the frame.
+  const clean = useMemo(() => sanitize(html), [html])
+  const srcDoc = useMemo(
+    // themeVersion is the entire reason this list exists. frameStyles()
+    // snapshots the CSS variables into the frame's <style>, and custom
+    // properties do not cross into an iframe, so without a rebuild the email
+    // keeps the palette it was born with while the rest of the app flips.
+    () => buildSrcDoc(clean, allowRemoteImages),
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+    [clean, allowRemoteImages, themeVersion],
+  )
 
   const measure = useCallback(() => {
     const frame = ref.current
@@ -171,7 +199,7 @@ export default function EmailBody({ html, maxHeight }) {
         onLoad={onLoad}
         // No allow-scripts. Ever. See the header comment.
         sandbox="allow-same-origin allow-popups allow-popups-to-escape-sandbox"
-        srcDoc={buildSrcDoc(clean, allowRemoteImages)}
+        srcDoc={srcDoc}
         style={{ height: maxHeight ? Math.min(height, maxHeight) : height }}
       />
     </div>
diff --git a/frontend/theme.test.mjs b/frontend/theme.test.mjs
new file mode 100644
index 0000000..4f6d8e5
--- /dev/null
+++ b/frontend/theme.test.mjs
@@ -0,0 +1,143 @@
+/**
+ * Theme-propagation test.
+ *
+ *   npm run test:theme
+ *
+ * Flipping [data-theme] repaints the entire app for free, because every colour
+ * in styles.css is a custom property. This test guards the one component that
+ * cannot ride that: EmailBody renders into an iframe, and custom properties do
+ * not inherit across a document boundary, so its palette is snapshotted into the
+ * frame's <style> and has to be rebuilt by hand.
+ *
+ * That made it the one place where a theme switch left stale colours on screen —
+ * on /inbox (both tabs) and /matching. The assertions below are what stops it
+ * regressing: a real MutationObserver, a real attribute flip, and the frame's
+ * own srcDoc read back afterwards.
+ */
+import { mkdtempSync, rmSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { pathToFileURL } from 'node:url'
+
+import esbuild from 'esbuild'
+import { JSDOM } from 'jsdom'
+
+// ---------------------------------------------------------------- environment
+const dom = new JSDOM('<!doctype html><html data-theme="light"><body><div id="root"></div></body></html>', {
+  url: 'http://localhost:5173/',
+  pretendToBeVisual: true,
+})
+
+globalThis.window = dom.window
+globalThis.document = dom.window.document
+Object.defineProperty(globalThis, 'navigator', { value: dom.window.navigator, configurable: true })
+globalThis.HTMLElement = dom.window.HTMLElement
+globalThis.Element = dom.window.Element
+globalThis.Node = dom.window.Node
+globalThis.DOMParser = dom.window.DOMParser
+globalThis.getComputedStyle = dom.window.getComputedStyle
+globalThis.localStorage = dom.window.localStorage
+globalThis.requestAnimationFrame = (cb) => setTimeout(() => cb(Date.now()), 0)
+globalThis.cancelAnimationFrame = clearTimeout
+globalThis.IS_REACT_ACT_ENVIRONMENT = true
+// jsdom ships a real MutationObserver, and this test depends on it — the render
+// smoke test stubs it out, which would silently make every assertion here pass.
+globalThis.MutationObserver = dom.window.MutationObserver
+
+// ---------------------------------------------------------------- bundle
+const outDir = mkdtempSync(join(tmpdir(), 'tf-theme-'))
+const outFile = join(outDir, 'entry.mjs')
+
+await esbuild.build({
+  stdin: {
+    contents: `
+      import { act } from 'react'
+      import { createRoot } from 'react-dom/client'
+      import EmailBody from './ui/EmailBody'
+
+      export async function mount(container, html) {
+        const root = createRoot(container)
+        await act(async () => { root.render(<EmailBody html={html} />) })
+        return root
+      }
+      export async function flush() { await act(async () => {}) }
+    `,
+    resolveDir: 'src',
+    sourcefile: 'theme-entry.jsx',
+    loader: 'jsx',
+  },
+  outfile: outFile,
+  bundle: true,
+  format: 'esm',
+  platform: 'node',
+  target: 'node20',
+  jsx: 'automatic',
+  loader: { '.js': 'jsx', '.jsx': 'jsx' },
+  logLevel: 'error',
+  define: {
+    'process.env.NODE_ENV': '"development"',
+    'import.meta.env': JSON.stringify({ DEV: true, PROD: false, MODE: 'development', VITE_API_BASE: '' }),
+  },
+})
+
+// ---------------------------------------------------------------- run
+let failed = 0
+const check = (label, ok, detail = '') => {
+  console.log(`${ok ? 'ok   ' : 'FAIL '} ${label}${ok || !detail ? '' : `\n      ${detail}`}`)
+  if (!ok) failed++
+}
+
+/** The palette a real stylesheet would supply, set inline so jsdom resolves it. */
+function applyPalette(theme) {
+  const root = dom.window.document.documentElement
+  const vars = theme === 'dark'
+    ? { '--bg-sunken': '#0e1d1f', '--text': '#e8f2ef', '--border': '#24403f', '--primary': '#ceff71' }
+    : { '--bg-sunken': '#f1f7f4', '--text': '#10231f', '--border': '#dbe8e2', '--primary': '#004d43' }
+  for (const [k, v] of Object.entries(vars)) root.style.setProperty(k, v)
+  root.setAttribute('data-theme', theme)
+}
+
+const srcDocOf = (container) => container.querySelector('iframe')?.getAttribute('srcdoc') || ''
+
+try {
+  const mod = await import(pathToFileURL(outFile).href)
+
+  applyPalette('light')
+  const container = dom.window.document.createElement('div')
+  dom.window.document.body.appendChild(container)
+  await mod.mount(container, '<p>Hello from a candidate.</p>')
+
+  const light = srcDocOf(container)
+  check('renders an iframe', Boolean(light), 'no iframe in the output')
+  check('light frame declares color-scheme: light', light.includes('color-scheme: light'))
+  check('light frame picks up the light background', light.includes('#f1f7f4'), light.slice(0, 200))
+  check('light frame picks up the light text colour', light.includes('#10231f'))
+  check('the mail itself still renders', light.includes('Hello from a candidate.'))
+
+  // The flip under test. MutationObserver delivers on a microtask, so the
+  // act() flush below is what lets the re-render land before we read back.
+  applyPalette('dark')
+  await mod.flush()
+  await mod.flush()
+
+  const dark = srcDocOf(container)
+  check('THE REGRESSION: frame rebuilds on theme flip', dark !== light,
+    'srcDoc is byte-identical after the flip — the email kept the light palette')
+  check('dark frame declares color-scheme: dark', dark.includes('color-scheme: dark'),
+    dark.slice(0, 200))
+  check('dark frame picks up the dark background', dark.includes('#0e1d1f'))
+  check('dark frame picks up the dark text colour', dark.includes('#e8f2ef'))
+  check('light colours are gone', !dark.includes('#f1f7f4') && !dark.includes('#10231f'))
+  check('the mail survives the rebuild', dark.includes('Hello from a candidate.'))
+
+  // And back, so this is a toggle rather than a one-way upgrade.
+  applyPalette('light')
+  await mod.flush()
+  await mod.flush()
+  check('flips back to light', srcDocOf(container).includes('color-scheme: light'))
+} finally {
+  rmSync(outDir, { recursive: true, force: true })
+}
+
+console.log(failed ? `\n${failed} theme check(s) failed` : '\nAll theme checks passed')
+process.exit(failed ? 1 : 0)