Design tweaks per #11540 and #11551

Design tweaks per the many discussions and proposals in #11540 and #11551. Including changed icon for Documents, improved colours, transparent tab bar, box shadow on tab panels for some depth etc. Props to the people that discussed these changes in the issues mentioned.

This does not yet include Christian's latest proposal in #11540, however it does introduce some new sass variables that would make implementing that a bit easier as well as tweaking other stuff. At time of coding there was no consensus yet on Chris' proposal though that could be the next iteration if that's what the community wants.