[Bug]: fix theme toggle visibility on hover state

### Consisely describe the issue

The sun icon is clearly visible in its default state with a white icon on the purple background. However, when hovering, the background becomes lighter while the icon remains white, resulting in poor contrast and reduced visibility.

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### LLM Policy

- [x] This issue is human-written, and folow up comments will also be human-written

The issue does not name a file, test, or entry point. Start by locating the theme toggle component and its hover styles, then compare the sun icon and hover background colors. Done means the icon remains clearly visible in both default and hover states, with the relevant frontend checks passing.

Written by the indexing model from the issue text.