Admin Reskin: Correct Customizer “Publish” button size.

Follow-up to [61646].

Props mohitbsf, wildworks, joedolson, audrasjb, huzaifaalmesbah, SergeyBiryukov.
Fixes #64689.

git-svn-id: https://develop.svn.wordpress.org/trunk@62295 602fd350-edb4-49c9-b593-d223f7449a82