diff --git a/electron/main.js b/electron/main.js
index 7e87b888..554996ac 100644
--- a/electron/main.js
+++ b/electron/main.js
@@ -3125,6 +3125,13 @@ ipcMain.handle('set-window-buttons-visible', (_e, visible) => {
   if (process.platform !== 'darwin' || !mainWindow || mainWindow.isDestroyed()) return;
   try { mainWindow.setWindowButtonVisibility(!!visible); } catch (err) { console.warn('[main] setWindowButtonVisibility failed:', err.message); }
 });
+// The creation-time backgroundColor is boot-dark; the renderer re-points it at the live theme's page
+// color so a live resize paints theme-matched filler, not a dark band behind a light UI.
+ipcMain.handle('set-window-background', (_e, color) => {
+  if (!mainWindow || mainWindow.isDestroyed()) return;
+  if (typeof color !== 'string' || !/^#[0-9a-fA-F]{6}$/.test(color)) return;
+  try { mainWindow.setBackgroundColor(color); } catch (err) { console.warn('[main] setBackgroundColor failed:', err.message); }
+});
 // Phase 2 provenance: the renderer's About panel shows the commit this build
 // was cut from, so a screenshot is enough to identify the exact code shipped.
 ipcMain.handle('get-build-info', () => getBuildInfo());
diff --git a/electron/preload.js b/electron/preload.js
index 65459774..3492415d 100644
--- a/electron/preload.js
+++ b/electron/preload.js
@@ -41,6 +41,8 @@ contextBridge.exposeInMainWorld('openswarm', {
   getAppVersion: () => ipcRenderer.invoke('get-app-version'),
   // Arc-style chrome: the mac traffic lights hide at rest; the dashboard's top-edge hover reveals them.
   setWindowButtonsVisible: (visible) => ipcRenderer.invoke('set-window-buttons-visible', visible),
+  // Native window bg tracks the theme so a live resize never paints the boot-dark color behind a light UI.
+  setWindowBackground: (color) => ipcRenderer.invoke('set-window-background', color),
 
   // Phase 2 provenance: { sha, shortSha, builtAt, channel } for the About panel.
   getBuildInfo: () => ipcRenderer.invoke('get-build-info'),
diff --git a/frontend/src/shared/styles/ThemeContext.tsx b/frontend/src/shared/styles/ThemeContext.tsx
index e6ecb367..6080b695 100644
--- a/frontend/src/shared/styles/ThemeContext.tsx
+++ b/frontend/src/shared/styles/ThemeContext.tsx
@@ -117,6 +117,9 @@ export const ThemeProvider: React.FC<{ children: React.ReactNode }> = ({ childre
 
   const tokens = useMemo(() => withAccent(mode === 'dark' ? darkTokens : lightTokens, accent, mode), [mode, accent]);
 
+  // The native window was created boot-dark; keep it on the theme's page color so live-resize filler never flashes dark under a light UI.
+  useEffect(() => { void window.openswarm?.setWindowBackground?.(tokens.bg.page); }, [tokens.bg.page]);
+
   // Stable identities: SettingsLoader's "apply settings.theme" effect lists setMode in its deps, so a setter that changed every render made that effect re-fire on each toggle and re-assert the OLD persisted theme until the debounced save caught up: live theme snapped back for ~900ms = the switch flicker.
   const toggleMode = useCallback(() => setModeState((m) => (m === 'light' ? 'dark' : 'light')), []);
   const setMode = useCallback((m: ThemeMode) => setModeState(m), []);
diff --git a/frontend/src/types/electron.d.ts b/frontend/src/types/electron.d.ts
index 9268e15b..290b3419 100644
--- a/frontend/src/types/electron.d.ts
+++ b/frontend/src/types/electron.d.ts
@@ -62,6 +62,7 @@ declare global {
     getWebviewPreloadPath: () => string;
     getAppVersion: () => Promise<string>;
     setWindowButtonsVisible?: (visible: boolean) => Promise<void>;
+    setWindowBackground?: (color: string) => Promise<void>;
     getBuildInfo: () => Promise<{ sha: string; shortSha: string; builtAt: string | null; channel: string }>;
     getUpdateStatus: () => Promise<{ status: string; info: any; error: string | null }>;
     getCrashRecoveryInfo?: () => Promise<{ ts: number; parent_pid: number; uptime_ms: number } | null>;