Themes: Avoid a JS error in the keyup event handler when the overlay is not yet initialized.

fixes #26497.

Built from https://develop.svn.wordpress.org/trunk@26826


git-svn-id: http://core.svn.wordpress.org/trunk@26713 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Themes: use explicit 'search installed themes' label and placeholder.

fixes #26496.

Built from https://develop.svn.wordpress.org/trunk@26827


git-svn-id: http://core.svn.wordpress.org/trunk@26714 1a063a9b-81f0-0310-95a4-ce76da25c4cd