The process is relatively straight forward. There is the Beginners Guide … to get an overview about the different types to modify themes.
The important part is at the end of the post, if the reader made it that far: Further Reading … Which tells us, what else is important. …
IMO the minimum requirement to start discourse theming is “Read it all at least once!”
One may say: I don’t have a discourse instance to create themes and I’m not an admin here to test stuff. … We are covered: Beginner's guide to using Discourse Themes - admins - Discourse Meta
There is https://discourse.theme-creator.io which will allow us to create and test themes, without the need to mess with an existing forum, where errors may be a catastrophic.
I did sign up there, to test the minimal theme, that is part of this forum, because of user requests. … The minimal theme did already exist. So I only needed to test it.
I don’t have any idea how to create a new theme and I don’t have the time to even read the docs linked here. Since the devil is in the details the discourse forum may be the best way to ask specific questions: https://meta.discourse.org/ … They are super friendly and well formulated questions get competent answers …
I think that’s it.
Why Doesn’t Discourse Look Like TiddlyWiki?
May be, because if we want to create a new theme, we need to know the underlaying system very well. …
IMO it’s just a question of “how far the commitment” goes.