Improve Bufferline theme

[uloco]

Bufferline theme is basically just what is used per default using the standard Tabline hl groups and could be improved.

When I tried the first time, the issue was with the underline as selected tab messing up everything when set manually. I need to theme all variants manually but then the icon colors don't auto change. Need to figure out how to do it.

Possible workaround would be, keep the select stuff as is and only theme the selected visible / window bg etc.