Summary

This PR improves the visibility of example components in dark theme across several CDK examples:

• Menu examples: Standalone trigger and nested context menus now have light backgrounds to ensure buttons are visible in dark mode.
• Overlay examples: Added color: CanvasText to ensure text is readable.
• Portal examples: Dashed borders changed from black to white for better contrast in dark theme.

Changes

• cdk-menu-nested-context-example.css: border color updated.
• cdk-menu-standalone-menu-example.css: background for .example-standalone-trigger added with hover/active states.
• cdk-overlay-basic-example.css: text color adjusted for dark theme.
• cdk-portal-overview-example.css: border color updated for dark theme.

Motivation

These changes make example components readable and visually consistent in dark mode without affecting light theme appearance.

Verification

Tested all updated examples in both light and dark themes to ensure proper visibility and styling.