Those 4 are the default ones that come with the software, people are already working on custom themes, in time we'll get a lot of options I'm sure.
For now you can get custom themes by using a browser addon like stylus and grabbing the themes from github, for example: https://github.com/2xx04/lemmy-ui-themes
I use this link to search https://userstyles.world/search?q=lemmy . I'm still waiting for a grid view since I suck at coding.
As time moves on, more and more fun features will come out. For now, I think everyone, including mobile developers, are focused on making everything solid before adding extras. Memmy’s IOS app has around 6 themes for your info!
I've got two on the go...both are a work-in-progress but are very usable as daily drivers:
Rediggit for Lemmy, which is also available as a native theme on the lemmy.ca instance:
I do my best to organize the stylesheet contents in a logical order, and to comment what each selector and declaration is doing, for those who may not know CSS very well.
Support / questions about Lemmy.