Admin Reskin: Standardize theme install overlay button height on mobile.

Follow-up to [61646].

Props abcd95, wildworks, mosescursor, sainathpoojary, SergeyBiryukov.
See #64999.

git-svn-id: https://develop.svn.wordpress.org/trunk@62294 602fd350-edb4-49c9-b593-d223f7449a82