Accessibility: Add missing label for the theme upload input.

See [11152] for plugins.

Props mercime.
Fixes #35113.
Built from https://develop.svn.wordpress.org/trunk@35958


git-svn-id: https://core.svn.wordpress.org/trunk@35922 1a063a9b-81f0-0310-95a4-ce76da25c4cd