Feature request: add dark mode support

Would be great to have a dark mode option. Many users prefer dark themes for accessibility and reduced eye strain.

Some suggestions:
- Auto-detect system preference
- Toggle in settings
- Respect `prefers-color-scheme`

Happy to help with a PR if there's interest.

The issue proposes dark mode with system-preference detection, a settings toggle, and `prefers-color-scheme` support, but names no files or entry points. First identify the user-interface and settings code, then define the supported behavior and verify that the selected theme and automatic preference detection work consistently.

Written by the indexing model from the issue text.