Following core base font-size of `0.875em` in Vector to align with
other elements on the page.
Bug: T175295
Change-Id: Id9c8652820641d6fa1c6b1dd06247967ba649f25
Replacing all colors with current WikimediaUI palette ones.
Also:
- changing two files over to LESS in order to
- make use of central gradient mixin instead of one with
unnecessary vendor extensions and
- concatenating selectors in one occurrence.
Bug: T174237
Change-Id: I5d302f29b079f6abdba5abf3b58d6ba254e1a4ac