Administration: Update Events widget background and border colors.

This changeset ensures the Events dashboard widget uses the new default admin colors.

Props r1k0, huzaifaalmesbah, mukesh27, audrasjb, amin7, shailu25, rishavdutta.
Fixes #64780.



git-svn-id: https://develop.svn.wordpress.org/trunk@62031 602fd350-edb4-49c9-b593-d223f7449a82