Theme variants & improvements

Decided to defer this for now.

- [ ] Choose variants
- [ ] Add config UI

Misc improvements:

- [ ] fix duplicate open-colors
- [ ] add scrollbar hover
- [ ] refactor to use more css vars, remove data-theme prefixing
- [ ] better popover separation for the dark theme
- [ ] dark timeseries hover styling
- [ ] better focus colors (focus on menu items is incorrect for example)
