MediaWiki theme: SelectFileWidget drop target aligned to UX patterns
MediaWiki theme: SelectFileWidget drop target aligned to UX patterns

Aligning SelectFileWidget with drop target to patterns elsewhere and
to WikimediaUI color palette.

Bug: T158027
Change-Id: I50377f2d3773257e4b89295e65852ee1dd1e8234

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