Add theme variables: --theme-fab-bg, --theme-fab-color, --offline-card-bg

Customizable via theme files:
- Dark: FAB uses accent-primary (#3f8ff0), card rgba(0,0,0,0.6)
- Light: FAB uses color-primary (#112855), card rgba(0,0,0,0.55)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/media/css/theme/dark.standard.css

@@ -827,6 +827,13 @@ color-scheme: dark;
 --backdrop-zindex: 1040;
 --backdrop-bg: hsl(0, 0%, 0%);
 --backdrop-opacity: 0.5;
+
+/* ===== THEME FAB ===== */
+--theme-fab-bg: var(--accent-color-primary, #3f8ff0);
+--theme-fab-color: #fff;
+
+/* ===== OFFLINE PAGE ===== */
+--offline-card-bg: rgba(0, 0, 0, 0.6);
 }
 
 .btn {

src/media/css/theme/light.standard.css

@@ -826,6 +826,13 @@ color-scheme: light;
 --backdrop-zindex: 1040;
 --backdrop-bg: hsl(0, 0%, 0%);
 --backdrop-opacity: 0.5;
+
+/* ===== THEME FAB ===== */
+--theme-fab-bg: var(--color-primary, #112855);
+--theme-fab-color: #fff;
+
+/* ===== OFFLINE PAGE ===== */
+--offline-card-bg: rgba(0, 0, 0, 0.55);
 }
 
 .btn {