diff --git a/packages/electron/main.mjs b/packages/electron/main.mjs
index 90f8106d..ba41017b 100644
--- a/packages/electron/main.mjs
+++ b/packages/electron/main.mjs
@@ -2450,6 +2450,13 @@ const createBrowserWindow = ({ label, restoreGeometry, url, runtimeConfig = {} }
       if (url.protocol === 'devtools:') return true;
       if (url.protocol === `${UI_PROTOCOL}:`) return true;
       if (url.protocol !== 'http:' && url.protocol !== 'https:') return false;
+      // In development the renderer is served by Vite while state.localOrigin
+      // remains the separate local API server. Permit same-origin reloads from
+      // the renderer itself so Vite full-reload fallbacks stay in Electron.
+      try {
+        if (new URL(browserWindow.webContents.getURL()).origin === url.origin) return true;
+      } catch {
+      }
       if (state.localOrigin) {
         try {
           if (new URL(state.localOrigin).origin === url.origin) return true;
@@ -4681,6 +4688,9 @@ const isLocalSender = (webContents) => {
     if (!raw) return false;
     const url = new URL(raw);
     if (url.protocol === `${UI_PROTOCOL}:` && url.hostname === 'app') return true;
+    // Electron dev renders from Vite while the local API is served on a
+    // separate port. This exact loopback HMR origin is trusted only in dev.
+    if (isDev && url.origin === `http://127.0.0.1:${process.env.OPENCHAMBER_HMR_UI_PORT || '5173'}`) return true;
     if (url.protocol !== 'http:' && url.protocol !== 'https:') return false;
     if (state.localOrigin) {
       try {
diff --git a/packages/ui/src/contexts/ThemeSystemContext.tsx b/packages/ui/src/contexts/ThemeSystemContext.tsx
index 7ddf2b3c..516676b4 100644
--- a/packages/ui/src/contexts/ThemeSystemContext.tsx
+++ b/packages/ui/src/contexts/ThemeSystemContext.tsx
@@ -19,6 +19,7 @@ import {
   DEFAULT_LIGHT_THEME_ID,
   DEFAULT_DARK_THEME_ID,
 } from '@/lib/theme/themes';
+import { withPrColors } from '@/lib/theme/themes/prColors';
 import { ThemeSystemContext, type ThemeContextValue } from './theme-system-context';
 import type { VSCodeThemePayload } from '@/lib/theme/vscode/adapter';
 import { runtimeFetch } from '@/lib/runtime-fetch';
@@ -158,6 +159,7 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
   const [preferences, setPreferences] = useState<ThemePreferences>(() => buildInitialPreferences(defaultThemeId));
   const [systemPrefersDark, setSystemPrefersDark] = useState<boolean>(() => getInitialSystemPreference());
   const [customThemes, setCustomThemes] = useState<Theme[]>([]);
+  const [developmentThemes, setDevelopmentThemes] = useState<Theme[]>([]);
   const [embeddedBootstrapTheme] = useState<Theme | null>(() => readEmbeddedCurrentTheme());
   const [embeddedSyncedTheme, setEmbeddedSyncedTheme] = useState<Theme | null>(null);
   const [customThemesLoading, setCustomThemesLoading] = useState(false);
@@ -204,10 +206,32 @@ export function ThemeSystemProvider({ children, defaultThemeId }: ThemeSystemPro
 
     // Custom themes first so they can override built-ins with the same id.
     customThemes.forEach(add);
+    // Vite publishes valid built-in JSON edits through this development-only
+    // runtime channel, avoiding a full page reload for theme work.
+    developmentThemes.forEach(add);
     themes.forEach(add);
 
     return merged;
-  }, [customThemes, embeddedBootstrapTheme, embeddedSyncedTheme, isVSCode, vscodeTheme]);
+  }, [customThemes, developmentThemes, embeddedBootstrapTheme, embeddedSyncedTheme, isVSCode, vscodeTheme]);
+
+  useEffect(() => {
+    const handleThemeHmr = (event: Event) => {
+      const theme = (event as CustomEvent<unknown>).detail;
+      if (!isValidTheme(theme)) return;
+
+      const nextTheme = withPrColors(theme);
+      setDevelopmentThemes((previous) => {
+        const index = previous.findIndex((candidate) => candidate.metadata.id === nextTheme.metadata.id);
+        if (index < 0) return [...previous, nextTheme];
+        const next = [...previous];
+        next[index] = nextTheme;
+        return next;
+      });
+    };
+
+    window.addEventListener('openchamber:theme-hmr', handleThemeHmr);
+    return () => window.removeEventListener('openchamber:theme-hmr', handleThemeHmr);
+  }, []);
 
   const getThemeByIdFromAvailable = useCallback(
     (themeId: string): Theme | undefined => availableThemes.find((theme) => theme.metadata.id === themeId),
diff --git a/packages/web/src/main.tsx b/packages/web/src/main.tsx
index 6d0a41a3..fea06385 100644
--- a/packages/web/src/main.tsx
+++ b/packages/web/src/main.tsx
@@ -125,6 +125,12 @@ const start = async (): Promise<void> => {
 
 void start();
 
+if (import.meta.hot) {
+  import.meta.hot.on('openchamber:theme-updated', (theme: unknown) => {
+    window.dispatchEvent(new CustomEvent('openchamber:theme-hmr', { detail: theme }));
+  });
+}
+
 if (import.meta.env.PROD) {
   registerPwaServiceWorker();
 } else {
diff --git a/packages/web/vite.config.ts b/packages/web/vite.config.ts
index 553a892b..2b1c4fe7 100644
--- a/packages/web/vite.config.ts
+++ b/packages/web/vite.config.ts
@@ -11,6 +11,31 @@ const packageJson = JSON.parse(readFileSync(path.resolve(__dirname, 'package.jso
 const pwaDevEnabled = process.env.OPENCHAMBER_DISABLE_PWA_DEV !== '1';
 const reactScanToggle = (process.env.VITE_ENABLE_REACT_SCAN ?? '').toLowerCase();
 const enableReactScan = reactScanToggle === '1' || reactScanToggle === 'true' || reactScanToggle === 'on' || reactScanToggle === 'yes';
+const themeDirectory = path.resolve(__dirname, '../ui/src/lib/theme/themes');
+
+const themeJsonHmrPlugin = () => ({
+  name: 'openchamber-theme-json-hmr',
+  handleHotUpdate({ file, server }: { file: string; server: { ws: { send: (payload: unknown) => void } } }) {
+    if (!file.startsWith(`${themeDirectory}${path.sep}`) || path.extname(file) !== '.json') {
+      return;
+    }
+
+    try {
+      server.ws.send({
+        type: 'custom',
+        event: 'openchamber:theme-updated',
+        data: JSON.parse(readFileSync(file, 'utf-8')),
+      });
+      // Theme JSON is applied by the runtime event listener. Returning no
+      // modules prevents Vite's otherwise unavoidable page-reload fallback.
+      return [];
+    } catch {
+      // Leave the previous valid theme active while an editor writes invalid
+      // or incomplete JSON; the next valid save will replace it.
+      return [];
+    }
+  },
+});
 
 export default defineConfig({
   root: path.resolve(__dirname, '.'),
@@ -39,6 +64,7 @@ export default defineConfig({
       },
     },
     themeStoragePlugin(),
+    themeJsonHmrPlugin(),
     VitePWA({
       strategies: 'injectManifest',
       srcDir: 'src',