Custom theme colors and Firefox dark mode not applying

[draguindin]

Theme customization appears to be non-functional. Tested:

1. Changed custom theme colors (background, text, all syntax colors), no change on new tabs or hard refresh
2. Enabled Firefox's native dark theme globally
3. Reinstalled extension
4. No console errors

The theme options panel exists and saves settings, but the colors don't actually apply to the JSON viewer. This may be related to issue #78 -- the theme settings UI appears disconnected from the actual CSS generation.

Environment: Firefox 140.12.0esr, JSON Lite 21.3.0