Hi Breeanna-
Thanks for the question!
In WordPress, you have to have at least one theme. So in order to remove the active theme, you should upload the theme you want to use, and activate it. Then, you’ll be able to remove the currently active theme.
However, if I understand your question, and please correct me if I’m wrong, it sounds like you’re trying to get rid of WordPress altogether and use a static HTML and CSS website?
If that’s the case, you can use your FTP client to delete the WordPress files and simply upload your HTML site into the public directory.