DropdownWidget: Add white background in MediaWiki theme
DropdownWidget: Add white background in MediaWiki theme

DropdownWidget merges with the background
when used with the MediaWiki theme-
https://phabricator.wikimedia.org/F180180

Change-Id: I5fe1a6a2ca34b42579a9164b81f986bf22b359ee

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