Customize: Fix alignment of Customize, Activate, and Live Preview buttons...

Customize: Fix alignment of Customize, Activate, and Live Preview buttons appearing on the Themes admin screen.

Amends [41893].
Props Mahvash-Fatima.
See #42215.
Fixes #42275.

Built from https://develop.svn.wordpress.org/trunk@41949


git-svn-id: https://core.svn.wordpress.org/trunk@41783 1a063a9b-81f0-0310-95a4-ce76da25c4cd