Earlier I wrote about having to manually install Gnome Shell themes because using the User Theme extension messes up Gnome Shell themes when using extensions. This is because of the CSS bug themes’ CSS get ignored causing them to look weird. This can be remedied by installing the fix CSS extension.
Download the extension here.
Extract the file and place the extracted folder in the ~/.local/share/gnome-shell/extensions/ folder of your home folder.
Restart Gnome Shell by pressing Alt + F2 and running r.
You can now use the User Theme extension again. To install and apply extra themes using the Gnome Tweak Tool, check out my post here.