Improve diff viewer color styling for better theme support (Vibe Kanban)

[stunningpixels]

Summary

Improved the diff viewer styling with better color consistency across light/dark themes and legacy/new design systems.

Changes

• Added theme-aware CSS variables: Introduced dedicated variables (--line-added, --line-removed, --line-unchanged, --line-number-color) for diff highlighting that adapt to:
  • Light and dark modes
  • Legacy and new design systems
• Simplified diff color definitions: Replaced complex color-mix() calculations with cleaner CSS variable references for better maintainability
• Improved visual distinction: Updated diff highlight colors for better readability:
  • Added lines: green tones (hsl 160°/130° depending on theme)
  • Removed lines: red tones (hsl 10°/12° depending on theme)
• Minor text adjustment: Slightly increased --text-normal lightness (16% → 20%) in light mode for improved readability

Files Modified

• frontend/src/styles/diff-style-overrides.css - Main diff styling changes
• frontend/src/styles/new/index.css - Text color adjustment

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This PR was written using Vibe Kanban