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

Fix bug #8882 -- 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`.  (Removed unneeded `!important` in the process).

Also, add a mousedown (`select:active`) state for dark mode similar to the one in light mode, based on how other dark mode buttons are styled.