Reconsolidate colors

I have a feeling that there are used too many different color variations...

Light theme:

Dark theme:

Many of them, especially in the dark theme, are very very close to each other.
I suggest to
- [x] drop all unused colors
- [x] merge colors which are the same on both themes (like the starred color)
- [ ] give more speaking identifiers than `material_grey_xxx`
- [x] merge `slide_up_panel_header_background_color` and `slide_up_panel_slider_background_color`
- [ ] and maybe most important: Check how many and which grey gradations we *really* need and limit the color palette to those