Use a brighter blue against dark backgrounds in the admin menu and toolbar.

Not only does this pass contrast guidelines, but it also visually appears closer to the original highlight blue used against light backgrounds.

props melchoyce. fixes #30323.


git-svn-id: https://develop.svn.wordpress.org/trunk@30313 602fd350-edb4-49c9-b593-d223f7449a82