Customize: Unstick filter pane in Theme selector

Prevents the filter pane from blocking the view of theme search results when it's open.
Users are now able to just scroll to view results.

Props rclations.
Fixes #42212.

Built from https://develop.svn.wordpress.org/trunk@41903


git-svn-id: https://core.svn.wordpress.org/trunk@41737 1a063a9b-81f0-0310-95a4-ce76da25c4cd