Hi, how do I make it so that users can only have access to 1 theme?
Not very disability friendly to do that. For myself I need to have a theme that does not set off my Scotopic Sensitivity Syndrome (sometimes known as Mears-Irlen Syndrome). It is a component of my dyslexia. I would instanstly give up on any Discourse install that inhibit my use.
In admin → customize → themes you need to uncheck
theme can be selected by users on all the themes you wish to disable.