Darker style focus/hover on active buttons

Darker style focus/hover on active buttons

Currently, hovering with mouse or setting focus on a button which is
active has same styling as on a non-active button. This results in
problems for keyboard users, who set focus on a toggle and activate it,
but cannot visually see that their action had any effect. Ditto for
mouse users hovering over a toggle and clicking it. This adds an
explicit additional style for focus/hover on active buttons.
Note that this does not address issues of browser focus remaining on a
button after a mouse click (e.g. #13971), as this will likely require
extra JavaScript to fix.