Add dark theme + extra customization options.

[wz-ml]

Customization Options:

1. [ ] Light/Dark theme
2. [ ] Adjustable font sizes and styles.
3. [ ] Options to rearrange or hide certain UI elements.

Implementation suggestions:

• A settings panel could be added under Advanced Settings in the extension dropdown.
• We can use prefers-color-scheme to match the user's system color scheme.