@Ar_D I’m in a similar situation myself (although I got started with the theme already). I’d recommend you check these topics. You might want to stick to the first three at first and once you get a better sense of what you are doing, dig into the last 2.
- Beginner's guide to using Discourse Themes
- Designer's Guide to getting started with themes in Discourse
- Theme Creator, create and show themes without installing Discourse!
- Get started with Theme Creator and the Theme CLI
- Developing Discourse Themes & Theme Components
You can get very far with the default functionalities of Theme Creator. I did give Theme CLI a try (first time setting up something as complex, I’m very much a n00b) and found it extremely useful for getting super fast feedback on any design changes. Overall, my extremely limited HTML/CSS have gotten me very far.