Redo VE toolbar styling

A lot of CSS has been moved upstream since this was first written.

It is also possible to simplify the resizing of the toolbar by just
changing the base font size as all sizing in VE is done with ems.

MF's overriding of box-sizing necessitates another override.

Bug: T85025
Change-Id: Ifa646fe392b86ab5c9f4aa03cc3df833e49eab7e
Merge "Redo VE toolbar styling"