diff --git a/docs/gotchas.md b/docs/gotchas.md
index 13e28a4..fc873be 100644
--- a/docs/gotchas.md
+++ b/docs/gotchas.md
@@ -49,6 +49,13 @@ touching the adjacent code.
   treatment is lost and the icon renders black on dark menu bars.
 - `ApplicationMenu` is macOS-only in our usage; accelerators on Windows are
   single-character only. ContextMenu exists on macOS+Windows, not Linux.
+- **Window-open flash prevention is three-layer** — remove any one and the
+  white/inverted blink returns: (1) windows are created `hidden: true` and
+  revealed by `showWhenReady` on the webview's first `dom-ready` (800 ms
+  fallback), (2) the bun preload bakes the persisted `data-theme`/tone/accent
+  attributes before first paint (`buildPreload`), (3) `ui/src/index.html`
+  carries an inline `<style>` with the html background per theme — its oklch
+  values MUST stay in sync with the `--background` tokens in styles.css.
 - Dev vs prod URL: `getMainViewUrl()` resolves the ng-serve dev server only
   when `DEV_SERVER_URL` is set AND the updater channel is `dev` AND the
   server answers a HEAD request; secondary windows must reuse the resolved
diff --git a/src/bun/editor-windows.ts b/src/bun/editor-windows.ts
index 64f0c06..54f6937 100644
Binary files a/src/bun/editor-windows.ts and b/src/bun/editor-windows.ts differ
diff --git a/src/bun/index.ts b/src/bun/index.ts
index 0396513..dbce496 100644
--- a/src/bun/index.ts
+++ b/src/bun/index.ts
@@ -14,7 +14,7 @@ import { migrateLegacyDataDir } from "./store/paths";
 import { SettingsStore } from "./store/settings";
 import { AppTray } from "./tray";
 import { TransferEngine } from "./transfers/engine";
-import { buildPreload } from "./windows";
+import { buildPreload, showWhenReady } from "./windows";
 
 const PROD_URL = "views://app/index.html";
 
@@ -56,6 +56,11 @@ const events = {
 const rpc = createAppRPC({ registry, engine, store, settings, edits, contextMenu, events });
 const url = await getMainViewUrl();
 
+// Loaded before any window exists: the persisted theme is baked into every
+// window's preload so the first paint is already themed (no flash). Kept
+// fresh via the saveSettings flow below.
+let currentSettings = await settings.load();
+
 const isMac = process.platform === "darwin";
 
 const win = new BrowserWindow({
@@ -73,16 +78,20 @@ const win = new BrowserWindow({
 	...(isMac
 		? { titleBarStyle: "hiddenInset" as const, trafficLightOffset: { x: 12, y: 13 } }
 		: {}),
-	// lets the UI pick the fused layout before first paint
-	preload: buildPreload("main"),
+	// lets the UI pick the fused layout + theme before first paint
+	preload: buildPreload("main", currentSettings),
 	rpc,
+	// revealed on first dom-ready — avoids the unthemed native flash
+	hidden: true,
 });
+showWhenReady(win);
 
 const editors = new EditorWindows({
 	registry,
 	settings,
 	edits,
 	url,
+	currentSettings: () => currentSettings,
 	mainFrame: () => win.getFrame(),
 	notifyMain: { pathRefresh: (info) => rpc.send.pathRefresh(info) },
 });
@@ -90,6 +99,7 @@ const manager = new ManagerWindow({
 	store,
 	settings,
 	url,
+	currentSettings: () => currentSettings,
 	mainFrame: () => win.getFrame(),
 	notifyMain: {
 		managerState: (info) => rpc.send.managerState(info),
@@ -101,6 +111,7 @@ const manager = new ManagerWindow({
 
 events.openEditor = (ctx) => editors.open(ctx);
 events.settingsChanged = (s) => {
+	currentSettings = s; // future windows' preloads bake the fresh theme
 	editors.broadcastSettings(s);
 	manager.broadcastSettings(s);
 };
@@ -199,7 +210,7 @@ events.sessionClosed = (info) => {
 	editors.notifySessionClosed(info);
 };
 events.connectionsChanged = () => void applyApplicationMenu(store);
-void settings.load().then((s) => engine.setConcurrency(effectiveConcurrency(s)));
+engine.setConcurrency(effectiveConcurrency(currentSettings));
 
 void applyApplicationMenu(store);
 ApplicationMenu.on("application-menu-clicked", (event) => {
diff --git a/src/bun/manager-window.ts b/src/bun/manager-window.ts
index 17c05fd..ccec9c8 100644
--- a/src/bun/manager-window.ts
+++ b/src/bun/manager-window.ts
@@ -13,6 +13,8 @@ export interface ManagerWindowDeps {
 	settings: SettingsStore;
 	/** The once-resolved dev/prod view url. */
 	url: string;
+	/** Live settings snapshot — baked into the window's preload (theme flash). */
+	currentSettings: () => AppSettings;
 	mainFrame: () => { x: number; y: number; width: number; height: number };
 	/** Late-bound senders targeting the MAIN window's webview. */
 	notifyMain: {
@@ -76,6 +78,7 @@ export class ManagerWindow {
 			title: "Connections",
 			url: this.deps.url,
 			mode: "manager",
+			settings: this.deps.currentSettings(),
 			frame: {
 				x: Math.max(0, main.x + (main.width - MANAGER_WIDTH) / 2),
 				y: Math.max(0, main.y + (main.height - MANAGER_HEIGHT) / 2),
diff --git a/src/bun/windows.ts b/src/bun/windows.ts
index 062397d..1054556 100644
--- a/src/bun/windows.ts
+++ b/src/bun/windows.ts
@@ -1,4 +1,5 @@
 import { BrowserWindow } from "electrobun/bun";
+import type { AppSettings } from "../shared/types";
 
 const isMac = process.platform === "darwin";
 
@@ -6,20 +7,61 @@ const isMac = process.platform === "darwin";
  * The preload globals every window gets. Single composition point: the UI
  * reads them in ui/src/app/platform.ts before first paint (window chrome and
  * root-component selection both depend on them).
+ *
+ * The persisted theme attributes are baked in here — mirroring the
+ * delete/set logic of SettingsService.applyX — so the FIRST paint is already
+ * themed (the inline <style> in ui/src/index.html consumes them). Without
+ * this, forced themes only apply after the async getSettings RPC and new
+ * windows flash the wrong background.
  */
-export function buildPreload(mode: "main" | "editor" | "manager", ctx?: unknown): string {
+export function buildPreload(
+	mode: "main" | "editor" | "manager",
+	settings: AppSettings,
+	ctx?: unknown,
+): string {
 	let script =
 		`window.__APP_PLATFORM=${JSON.stringify(process.platform)};` +
 		`window.__APP_MODE=${JSON.stringify(mode)};`;
 	if (ctx !== undefined) script += `window.__APP_CTX=${JSON.stringify(ctx)};`;
+
+	const dataset: Record<string, string> = {};
+	if (settings.theme !== "system") dataset["theme"] = settings.theme;
+	if (settings.darkTone !== "dark") dataset["darkTone"] = settings.darkTone;
+	if (settings.accent !== "graphite") dataset["accent"] = settings.accent;
+	if (settings.textSize !== "default") dataset["text"] = settings.textSize;
+	for (const [key, value] of Object.entries(dataset)) {
+		script += `document.documentElement.dataset[${JSON.stringify(key)}]=${JSON.stringify(value)};`;
+	}
 	return script;
 }
 
+/**
+ * Windows are created hidden and revealed on the webview's first dom-ready —
+ * showing them earlier paints the native (white) surface before the themed
+ * page exists. The fallback timer guarantees a missed event can never leave
+ * a window invisible.
+ */
+export function showWhenReady(win: BrowserWindow): void {
+	let shown = false;
+	const show = () => {
+		if (shown) return;
+		shown = true;
+		win.show();
+	};
+	const fallback = setTimeout(show, 800);
+	win.webview.on("dom-ready", () => {
+		clearTimeout(fallback);
+		show();
+	});
+}
+
 export interface SecondaryWindowOptions {
 	title: string;
 	/** The once-resolved dev/prod url (see getMainViewUrl in index.ts). */
 	url: string;
 	mode: "editor" | "manager";
+	/** Current persisted settings — baked into the preload for flash-free paint. */
+	settings: AppSettings;
 	ctx?: unknown;
 	frame: { x: number; y: number; width: number; height: number };
 	/** Per-window rpc instance. NEVER share one across windows — each
@@ -36,16 +78,19 @@ export interface SecondaryWindowOptions {
  * native titlebar and the UI renders a compact non-drag header instead.
  */
 export function openSecondaryWindow(opts: SecondaryWindowOptions): BrowserWindow {
-	return new BrowserWindow({
+	const win = new BrowserWindow({
 		title: opts.title,
 		url: opts.url,
 		frame: opts.frame,
 		...(isMac
 			? { titleBarStyle: "hiddenInset" as const, trafficLightOffset: { x: 12, y: 13 } }
 			: {}),
-		preload: buildPreload(opts.mode, opts.ctx),
+		preload: buildPreload(opts.mode, opts.settings, opts.ctx),
 		rpc: opts.rpc,
+		hidden: true,
 	});
+	showWhenReady(win);
+	return win;
 }
 
 /**
diff --git a/ui/src/index.html b/ui/src/index.html
index dd444be..db3bd6c 100644
--- a/ui/src/index.html
+++ b/ui/src/index.html
@@ -6,6 +6,21 @@
     <base href="/" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <link rel="icon" type="image/x-icon" href="favicon.ico" />
+    <!-- Pre-stylesheet paint: the window background must be right BEFORE
+         styles.css parses or new windows flash white/inverted. Only the
+         html background decision lives here — values must stay in sync with
+         the corresponding --background tokens in styles.css. The data-*
+         attributes are baked in by the bun preload (buildPreload in
+         src/bun/windows.ts) so forced themes are correct at first paint. -->
+    <style>
+      html { background: oklch(1 0 0); }
+      html[data-theme='dark'] { background: oklch(0.245 0.005 285.9); }
+      html[data-theme='dark'][data-dark-tone='darker'] { background: oklch(0.141 0.005 285.823); }
+      @media (prefers-color-scheme: dark) {
+        html:not([data-theme='light']) { background: oklch(0.245 0.005 285.9); }
+        html:not([data-theme='light'])[data-dark-tone='darker'] { background: oklch(0.141 0.005 285.823); }
+      }
+    </style>
   </head>
   <body>
     <app-root></app-root>