I have seen many questions about enabling a GNOME Shell theme using gnome-tweak-tool that go like this:
I have the user-theme extension installed and turned on in Gnome Tweak Tool. I’ve unzipped the theme folder to /usr/share/themes and selected my gtk theme fine. However under Shell Theme, there’s still a “!” and it doesn’t allow me to do anything as far as selection. (It’s greyed out)
The caution sign is sometimes referred to as an exclamation mark by users.
The best workaround for this is to enable your theme from the command lien using gsettings as follows
gsettings set org.gnome.shell.extensions.user-theme name 'your theme name'
where your theme-name is the full name of your theme – which is installed under $HOME/.themes.
An alternative way is to edit this particular gsetting value using dconf-editor.