Media: Update upload file overlay colors.

Update the colors used for the file upload overlay mask to use the new admin theme colors.

Props opurockey, huzaifaalmesbah, wildworks, audrasjb, manhar, joedolson.
Fixes #65001.

git-svn-id: https://develop.svn.wordpress.org/trunk@62199 602fd350-edb4-49c9-b593-d223f7449a82