fix: dark theme as default in :root — no data-theme dependency

- Added full dark theme variables to bare :root selector (no attribute required)
- This ensures dark theme always loads even if data-theme is not set or stripped
- :root[data-theme='light'] still overrides for light mode
- Popout windows now always get dark theme baseline

frontend/style.css

@@ -17,7 +17,49 @@
   --err: #f87171;
 }
 
-/* ===== THEME — DARK (default) ===== */
+/* ===== THEME — DARK (fallback default, no data-theme) ===== */
+:root {
+  --bg-primary: #0f1117;
+  --bg-secondary: #161b22;
+  --bg-sidebar: #13161d;
+  --bg-hover: #1f2430;
+  --border: #21262d;
+  --text-primary: #e6edf3;
+  --text-secondary: #8b949e;
+  --text-muted: #484f58;
+  --accent: #58a6ff;
+  --accent-green: #3fb950;
+  --tag-bg: #1f2a3a;
+  --tag-text: #79c0ff;
+  --code-bg: #161b22;
+  --search-bg: #21262d;
+  --scrollbar: #30363d;
+  --resize-handle: #30363d;
+  --overlay-bg: rgba(0, 0, 0, 0.5);
+  --danger: #ff7b72;
+  --danger-bg: #3d1a18;
+  --success: #3fb950;
+  --success-bg: #1a3d1f;
+  --accent-card: #58a6ff;
+  --accent-bg: #1f2a3a;
+  --accent-text: #79c0ff;
+  --green: #3fb950;
+  --green-bg: #1a3d1f;
+  --purple: #a371f7;
+  --purple-bg: #2d1b4e;
+  --surface: #161b22;
+  --surface2: #0f1117;
+  --surface3: #0d0f14;
+  --border-md: #30363d;
+  --text: #e6edf3;
+  --text-2: #8b949e;
+  --text-3: #484f58;
+  --mono: "JetBrains Mono", monospace;
+  --radius: 6px;
+  --radius-lg: 10px;
+}
+
+/* ===== THEME — DARK (explicit) ===== */
 :root[data-theme="dark"] {
   --bg-primary: #0f1117;
   --bg-secondary: #161b22;