fix: improve sidebar button visibility and hover effects in both ligh…

[tharunika-19]

Description

Improves the visibility of the right sidebar format/preset buttons in both light and dark mode.
Previously, inactive buttons had very low contrast borders and backgrounds that blended into
the sidebar, making them hard to distinguish.

Changes made:

• Replaced var(--border) and var(--surface) CSS variables with theme-aware Tailwind classes
• Added dark: prefix classes so borders and backgrounds work correctly in both light and dark mode
• Inactive buttons now have clearly visible borders and backgrounds in both themes

Related Issue

Fixes #1513

Type of Contribution

• Bug fix
• New feature
• Documentation update
• Refactor
• GSSoC contribution

Participant Info

• GitHub username: tharunika-19
• Contribution level (Beginner/Intermediate/Advanced): Beginner

Screen shots

Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

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[github-actions]

👋 Thanks for your PR, @tharunika-19!

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🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

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Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
• Build passes (bun run build)

Useful links

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Happy coding! 🎉

[github-actions]

✅ PR Format Check Passed — @tharunika-19

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This does not mean the PR is approved — it just means the format is correct.