Lemmy has a list of themes. I like most of them but some of them are weird like the notepad one(called sketch). But I think everyone either uses the dark or light theme. I am not requesting the team to remove all the themes, maybe some of them can be removed?
If you are lemmur app, then you only get dark and light. Use the web app/lemmy.ml site to get access to these themes. Its in your profile page.