Meta: Design Kit improvements

TODO:
- [ ] Check if we can close https://github.com/vector-im/element-ios/issues/4356
- [ ] Dark theme: Each grey component should use Tertiary color: https://www.figma.com/file/X4XTH9iS2KGJ2wFKDqkyed/Compound?node-id=1237%3A16596
- [ ] Floating button: Keep drop shadow on light theme and remove it from on dark theme https://www.figma.com/file/X4XTH9iS2KGJ2wFKDqkyed/Compound?node-id=1972%3A25186
- [ ] ThemeService: Remove `isCurrentThemeDark` and put this boolean in `Theme`. Or find a way to use themed image without boolean.
- [ ] Colors: Use the right color names as defined here: https://www.figma.com/file/X4XTH9iS2KGJ2wFKDqkyed/Compound?node-id=1521%3A23026
- [ ] Check if actions highlighted state is defined in Figma or ask for it

Review the checklist and the linked Figma specifications, including the referenced issue 4356, before locating the relevant Design Kit and ThemeService areas. Done means resolving each checklist item or documenting the requested decision, including the dark-theme colors, floating-button shadow, theme handling, color names, and actions highlighted state.

Written by the indexing model from the issue text.