[eric] theme: default to light + fix light/dark switch flicker

backend/apps/settings/models.py

@@ -39,7 +39,7 @@ class AppSettings(BaseModel):
     default_max_turns: Optional[int] = None
     default_thinking_level: Literal["off", "low", "medium", "high", "auto"] = "auto"
     zoom_sensitivity: float = 50.0
-    theme: str = "dark"
+    theme: str = "light"
     # Shared across App Builder workspaces (each runs its own vite port / localStorage origin); null = follow system.
     app_template_theme_override: Optional[Literal["light", "dark"]] = None
     new_agent_shortcut: str = "Meta+l"

frontend/src/shared/state/settingsSlice.ts

@@ -123,7 +123,7 @@ export const DEFAULT_SETTINGS: AppSettings = {
   default_max_turns: null,
   default_thinking_level: 'auto',
   zoom_sensitivity: 50,
-  theme: 'dark',
+  theme: 'light',
   new_agent_shortcut: 'Meta+l',
   anthropic_api_key: null,
   browser_homepage: 'https://duckduckgo.com',

frontend/src/shared/styles/ThemeContext.tsx

@@ -1,4 +1,4 @@
-import React, { createContext, useContext, useState, useEffect, useMemo } from 'react';
+import React, { createContext, useContext, useState, useEffect, useMemo, useRef, useCallback } from 'react';
 import { ClaudeTokens, lightTokens, darkTokens } from './claudeTokens';
 
 type ThemeMode = 'light' | 'dark';
@@ -17,10 +17,20 @@ function getInitialMode(): ThemeMode {
     const stored = localStorage.getItem(STORAGE_KEY);
     if (stored === 'light' || stored === 'dark') return stored;
   } catch {}
-  if (window.matchMedia?.('(prefers-color-scheme: dark)').matches) return 'dark';
+  // Default light regardless of OS; the real choice arrives from settings a beat later, and matching that default here keeps the pre-load frame from flashing to the opposite theme.
   return 'light';
 }
 
+// A theme swap repaints every element, and the app's hover transitions (transition: background-color / all) would each crossfade the new color at their own duration: that staggered animation IS the flicker. Kill all transitions for the single swap frame so the change is one instant cut; hovers animate again next tick.
+function p_suppressTransitionsForSwap(): () => void {
+  const killer = document.createElement('style');
+  killer.appendChild(document.createTextNode('*,*::before,*::after{transition:none!important}'));
+  document.head.appendChild(killer);
+  void document.body.offsetHeight; // force a synchronous restyle so the rule applies to this frame
+  const id = window.setTimeout(() => killer.remove(), 0);
+  return () => { window.clearTimeout(id); killer.remove(); };
+}
+
 const ThemeContext = createContext<ThemeContextValue>({
   mode: 'light',
   tokens: lightTokens,
@@ -30,17 +40,21 @@ const ThemeContext = createContext<ThemeContextValue>({
 
 export const ThemeProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
   const [mode, setModeState] = useState<ThemeMode>(getInitialMode);
+  const firstMount = useRef(true);
 
   useEffect(() => {
     try { localStorage.setItem(STORAGE_KEY, mode); } catch {}
+    if (firstMount.current) { firstMount.current = false; return; }
+    return p_suppressTransitionsForSwap();
   }, [mode]);
 
   const tokens = useMemo(() => (mode === 'dark' ? darkTokens : lightTokens), [mode]);
 
-  const toggleMode = () => setModeState((m) => (m === 'light' ? 'dark' : 'light'));
-  const setMode = (m: ThemeMode) => setModeState(m);
+  // 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), []);
 
-  const value = useMemo(() => ({ mode, tokens, toggleMode, setMode }), [mode, tokens]);
+  const value = useMemo(() => ({ mode, tokens, toggleMode, setMode }), [mode, tokens, toggleMode, setMode]);
 
   return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
 };