I’m not sure if this actually has any effect on the number of themes or the willingness of other people to make them, but just from my own experience, the structure of Discourse’s HTML markup and CSS classes isn’t very well architected for easy or efficient theming.
If you want to make CSS-only theme changes, for the most part they’ll be tightly coupled to the forum’s markup and classes (that the theme doesn’t own unless you touch the HTML, too), and simple tweaks often take more work than they should, e.g. because there’s no single selector for them. (For example, the line separating posts in a topic isn’t actually a single border, it’s split between the avatar component and the post body. You can’t simply tweak it as you have to take into account various other elements that interact with it in non-obvious ways.)
For me personally this means that I’m only willing to make very minimal changes that don’t try to change too much, as I’d be worried that future forum updates would break my changes and it’d be hard to track down how to fix them.