improvement: support deeply-nested theme colors

[rellen]

Theme colors in tailwind can be deeply-nested, e.g. "my_theme": {"neutral": {"gray": "#DDDDDD", ...}}

Originally, these classes were not surfaced by Tails so it was not able to merge classes using them with other color-related classes.

Tails.Colors.all_color_classes can be changed to support these deeply-nested themes so that they are surfaced correctly.

Contributor checklist

• [x] Bug fixes include regression tests
• [x] Chores
• [ ] Documentation changes
• [x] Features include unit/acceptance tests
• [x] Refactoring
• [ ] Update dependencies

[zachdaniel]

🚀Thank you for your contribution!🚀