Elegant recolorable borders

That don't rely on hardcoded colors in assets.
Don't use assets for backdrop borders
Fix level bar troughs
Remove trough border asset usage
Don't use border assets for primary toolbars
Move all border styling into main css file
Try to fix linked items
fix spin buttons
fix scale trough
Fix selected combobox entry colors
Fix prelight on combobox button
Fix switch and scale background and border
Flatten switch active background style
fix column header buttons
Rename switch active trough color
Remove dark overrides that use border assets
Merge separate backgrounds file into main css
Fix window close button
Use a color variable instead of white for the inset highlights