Fix bug #8882 (Background flickers when opening themes dropdown) - ensure
the 'select:active > option' colors exactly match the 'select > option'
colors by explicitly specifying '> option' colors instead of letting them
be quasi-inherited from the parent 'select'.

Also, add a mousedown (:active) state for dark mode similar to the one in
light mode, based on how other dark mode buttons are styled.
Merge pull request #9140 from adobe/pflynn/select-dropdown-flicker

Fix bug #8882 (Background flickers when opening themes dropdown)