feat: add tooltips to theme selector buttons

[Anahskal]

[Feature]: Add tooltips to theme selector buttons

🎯 What issue does this address?
Adds tooltips to all theme selector buttons to improve usability. Tooltips display correctly in both light and dark modes.

✨ What does this change?

• index.html: Added tooltip markup for each theme button.
• css/themes.css: Added styles to display tooltips for all themes.

Checklist:

• Tooltip functionality added for all theme buttons.
• Works in both light and dark modes.
• Responsive and accessible.
• No unrelated files modified.

🖼️ Screenshots

Before Changes

After Changes

✅ Checklist

[x] I have read the project's [CONTRIBUTING.md] guidelines.
[x] My code follows the project's coding style and conventions.
[x] I have performed a self-review of my own code.
[x] My changes generate no new errors or warnings in the browser console.
[x] I have tested my changes locally on the Vercel-simulated environment (if applicable).
[x] For UI changes, I have tested on different screen sizes (responsive design check).
[x] My commit message(s) are clear and descriptive.

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Project	Deployment	Preview	Comments	Updated (UTC)
3d-blockstack	Ready	Preview	Comment	Oct 30, 2025 2:38pm

[Anahskal]

@maitri-vv kindly review my PR