Thanks for the feedback @Taleman.
My immediate reaction to this was that you're probably right, but at the same time, even when browsing themes on ThemeForest for Wordpress, I always think to myself that I should just write my own.
I think the results of your search will improve as the number of themes increases...since the fact that you're not finding what you need might just be a matter of it not being there.
Where do you see this theme file living? If it's in the theme itself, what about changes to the upstream? I think this may seem like a good idea at first, but the idea of theme-specific configuration files (i.e., rather than just having the author include a good
README.md [and we are improving this area of the documentation]) actually makes the theming capabilities less flexible since you'd be tethering the output and content of your site to a single theme rather than treating all your content and configuration as something separate.
You could go the route of a having a much stricter, really long set of site config variables, but I'm not sure how you could account for every type of site. Plus, this would be a nightmare for international users who speak English as a second language. You could make the data/content model more narrowly scoped, but then you have a Wordpress problem (i.e, everything is a "post" or "page"). I agree that these things should probably be easier, so I'm throwing out some ideas
The new docs are improved but far from perfect. I'll certainly continue to iterate and add to the installing and customizing pages under "Themes." If you can provide me some more specific feedback or even open an issue, I'll do what I can.
I empathize with you, but the truth is that a "tutorial explaining how to use hugo to create a nontrivial website" will likely never be part of documentation. This isn't a Hugo-specific sensibility. In the couple dozen SSG projects I looked at during my content analysis, almost none have any real substantive tutorials in them (outside of a few impressive Quick Starts). This is why so many OS projects have cottage industries built on top of full-length tutorials for beginners. For example, I spent $80 on a React Tutorial a year ago because the docs weren't getting me quite as deep into what I wanted to figure out.
In fact, https://hugodocs.info doesn't have a "tutorials" section at all, and this is for good reason: Hugo is changing (read: improving) constantly, and the old tutorials section was, quite literally, outdated in every single page. Instead, I think it's best to structure the new version as logically as possible, promote cross-linking, be fastidious about documenting undocumented (but still supported) and new features, and provide examples, examples, and more examples. Maintaining a comprehensive tutorial would be nearly impossible with current staffing, and also just an ineffective means of managing content for this type of site.
Hopefully the new and continually improved docs help you in this effort, but I think part of the appeal of Hugo is its flexibility.
NB, I plan on creating an entirely new quick start as soon as the community can come up with a default "theme" for Hugo, but I don't think it will be part of the first launch of the new docs site. The current Quick Start was great for v15, but it's way too end-to-end (speaking to your point of tutorials) and walks through what I see as a less-than-popular use case.
I'm not 100% sure on what you're asking, but I believe the answer is just "no." For one thing, Hugo doesn't deal in matters of CSS at all, at least currently.
Keep coming back to the forums and reaching out to the community. I'm sure someone will be able to help you