It’s probably easier to start with themes. So I guess you’d start at the theme development topics like Developer’s guide to Discourse Themes and Beginners’ guide to using Theme Creator and Theme CLI to start building a Discourse theme and Structure of themes and theme components. And then, I’d find a javascript book that makes sense to you. You’ll need to find one for people who don’t know programming, so it’s not the one that I’d want (I knew more then five languages in the late 1980s, which has made javascript a challenge for me in ways that it might not for you, while also simultaneously making it much easier).
For me the rails end came much faster, but most of whet I think you’d want to do you can do strictly on the front end, and it’s just much faster to get in from that side.