Themes: Add missing square bracket in jQuery selector.

Fixes a bug in Safari where the UI wasn't updated after deleting a theme due to
the incomplete selector.

Props Hristo Sg.
Fixes #39246.

Built from https://develop.svn.wordpress.org/trunk@39586


git-svn-id: https://core.svn.wordpress.org/trunk@39526 1a063a9b-81f0-0310-95a4-ce76da25c4cd