When installing themes manually as a user using the Gnome theme manager, themes get deployed when put into immediate effect, but do not get added to the list in gnome-appearance-settings. If you later-on switch to a system-wide theme, you have no option to return to your newly installed one (as it's not in the list).

When trying to reinstall the theme, the following things happen:

When using the theme installer (invocated e.g. from Firefox after downloading a theme file from art.gnome.org): "this file does not seem to be a valid theme file" (even though it obviously is, as it was "successfully" installed earlier and worked when set into immediate effect instead of opting to have it added to the list, what fails without warnig).

When using gnome-appeareance-settings and selecting the the .tar.gz on disk, the following message gets shown: "Installation for theme 'ThisIsTheFancyThemeIwant' failed." - "Can't move directory over directory". Still, while it might benefit the techy user, this does not help Joe Random.

There is a bug message on Ubuntu launchpad about the same issue:
https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/452279