Ik heb van Malcolm een heel bericht gekregen over jouw probleem.
Moet ik dat hier posten en vertalen of ben je nu geheel tevreden. Ik zal in ieder geval de Engelse tekst hieronder zetten. Als je dat wilt ga ik poroberen e.e.a. toe te lichten.
If user theme extension isn't installing, that is usually a missing

The user also needs to check if the theme requires a gtk2 or gtk-3
theme engine installed, there are two others oxygen and unico for gtk3.

If you can ask the user which theme they are trying to install and
provide a link.

Also ensure after adding a theme you need to restart the shell press
alt+F2 enter r and press enter key.

You can also run looking glass ( abuilt in debugger tool), press alt+F2
and enter lg press enter on the top there is an 'extensions' option,
when finished with looking glass press esc key to exit.