This is the toggle you’re referring, to right? Sidebar Theme Toggle
Can you clarify what you mean by “not visible at all”?
Perhaps you can share a screenshot to help illustrate?
I’m curious to learn more about your use case as well here.
Do you expect people to have a preference and generally stick with the theme they choose (and that you need to help them discover they can do that at all)?
Or is it that you expect people to be able to switch back and forth regularly, (e.g. depending on what content they are looking at)?
Or something else?