Add dark/light theme toggle for better accessibility

The dark theme is nice but sometimes I need light mode, especially during daytime.

**Current issue:**
- Only dark theme available
- Hard to use in bright environments
- No user preference saving

**What would help:**
- Toggle button to switch themes
- Remember my preference
- Light theme that's easy on eyes
- Smooth transition between themes

Some people prefer light themes and this would make the app more accessible.

No files, tests, or entry points are named in the issue. Start by locating the existing dark-theme styles and theme-related UI, then verify the toggle, saved preference, readable light colors, and transition behavior; done means both themes can be selected and the choice persists.

Written by the indexing model from the issue text.