Fixes#36296
## Summary
Add custom checkbox styling with proper contrast for Peek markdown preview in both
light and dark themes. The default browser checkbox appearance had poor contrast,
making checkboxes difficult to see.
## Changes
- Add CSS rules for input[type='checkbox'] in both HtmlLightHeader and HtmlDarkHeader
- Use SVG checkmark for checked state with white tick on blue background
- Light theme: #0366d6 accent color with #fff background
- Dark theme: #58a6ff accent color with #1e1e1e background
- Addresses WCAG 2.1 AA accessibility concern about checkbox visibility
758c3ba608