If the sound themes option in the Gnome volume control is disabled, it may be because the "gnome-settings-daemon" process is not running. The "gnome-settings-daemon" process is responsible for managing the settings and preferences of the Gnome desktop environment, including the sound settings.
To check if the "gnome-settings-daemon" process is running, use the ps command:
ps -ef | grep gnome-settings-daemon
If the "gnome-settings-daemon" process is not running, you can start it by running the following command:
gnome-settings-daemon
If the "gnome-settings-daemon" process is already running, you may need to reset the Gnome desktop settings to fix the problem. To reset the Gnome desktop settings, you can use the gnome-tweaks utility.
To open gnome-tweaks, click on the Activities menu in the top left corner of the screen, then type "Tweaks" in the search bar and click on the "Tweaks" icon. In the "Tweaks" utility, go to the "Extensions" tab and click on the "Reset to Defaults" button.
I hope this helps fix the problem with the disabled sound themes in the Gnome volume control. Let me know if you have any further questions.