fix: dark mode (Timeline, Activity, Raw Data)

[BelKed]

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:rocket: This description was created by Ellipsis for commit 031505b75f8461c9d07661e115a4aba7362d359d

Summary:

This PR enhances dark mode in the ActivityWatch web UI by updating styles for better visual consistency and readability.

Key points:

• Update dark mode styles in /static/dark.css
• Adjust background colors and fills for better visibility
• Modify sort icons for clarity in dark mode

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Generated with :heart: by ellipsis.dev

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 26.15%. Comparing base (854b116) to head (031505b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #575 +/- ## ======================================= Coverage 26.15% 26.15% ======================================= Files 27 27 Lines 1629 1629 Branches 280 280 ======================================= Hits 426 426 Misses 1145 1145 Partials 58 58 ```

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[ErikBjare]

Awesome, thanks!