fix dark/light mode surface colors and Remove button color

- Fix ContentViewer background invisible against card in dark mode
- Fix Remove button not red in dark mode by using color="red.6"
- Add distinct card background for light mode (gray-0) vs app background
- Use CSS custom properties for card background across color schemes

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

clients/web/src/theme/Card.ts

@@ -10,4 +10,7 @@ export const ThemeCard = Card.extend({
     if (props.variant === "responsive") return { root: "card-responsive" };
     return {};
   },
+  styles: () => ({
+    root: { backgroundColor: "var(--inspector-surface-card)" },
+  }),
 });