Apply mediawiki.ui.button colors to all skins

Specific skins can still add overrides using skinStyles.  However,
before this change, there was no custom style for any other skin.  They
just looked like Vector without the colors and type.  That grey could
be confused with a 'disabled' state.

Vector still has an override just for the font size and height.

Bug: 55554
Change-Id: I5ed8f6ca7d756731639fa4b62fbb4e4d74972100
Merge "Apply mediawiki.ui.button colors to all skins"