When I entered 'book cover design' into google the first thing the popped up was canva.com
The site lets you design a cover and at the end you simply buy the photos or use of them from the photographer who uploaded them. You can browse their templates and while there were a lot I did not see many any that fell into what I would see as urban fantasy or science fiction. IF I could find photos or designs of my own to use I could upload them. So it doesn't completely rule them out.
The next site was the website "Creative Penn". This isn't a design site but an article on the topic. The author of the site Joanna Penn a thriller author with New York Times best sellers, suggests in her article that you get a professional designed book cover, especially if you are a new author. While not the same genre, she has a point. A cover of a book is what catches the person's eyes enough to get them to read the blurb or even buy the book. I know for an author I have never read before the cover is what catches my attention first. Ms. Penn also offers courses that I plan to look at later.