Apex theme: DropdownWidget align CSS code to WikimediaUI theme
Apex theme: DropdownWidget align CSS code to WikimediaUI theme

This cleans up code and fixes background-color bleeding over the handle.

Bug: T183271
Change-Id: I5a73dc317cb5158b68baa1f7089bac49f41610f3

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui