Themes: Ensure the Activate button in theme preview overlay on Add Themes...

Themes: Ensure the Activate button in theme preview overlay on Add Themes screen has a consistent height.

Props afercia, pravinparmar2404, Toru, mukesh27.
Fixes #50316.
Built from https://develop.svn.wordpress.org/trunk@48143


git-svn-id: https://core.svn.wordpress.org/trunk@47912 1a063a9b-81f0-0310-95a4-ce76da25c4cd