I was addressing your second bullet point, rather than the first. The second seems to betray a bit of the misunderstanding I expressed:
To me, the first point is like saying ‘make’ is passing the buck because it doesn’t include the code for setting up UIs, and the second is like complaining that the documentation for a ‘C’ compiler like GCC doesn’t convey how to do a ‘real’ UI for prompting for user input.
Now, the first major bullet point and the diagrams you show are interesting, but the chances of keeping such diagrams fully specific and complete is about nil. Hugo is semver 0.xx.xx which means it is still in development and there are aspects of the Hugo’s processing that are still subject to change.
That isn’t to say a high-level approximate overview would not help a number of users, but it is important to remember that such a diagram would be informative not normative.
Now this type of diagram, and the kind of expanded ‘hello-world’ tutorial you mention (of which I think @zwbetz has done a good job of a basic version in the link you posted) would undoubtedly be beneficial new users of Hugo (with caveats I mention below), but it takes someone with time, energy,and interest to work on it. Hugo is not a commercial product and has no deep pocket funding for ‘productizing’ Hugo (which includes the kinds of documentation that seems to be looked for), so attempting to browbeat or complain documentation into existence won’t work (not suggesting that is what you are trying to do @bdpp, but others seem to be trying to take that approach), so making the ‘new user’ experience more comfortable has to be done by someone who likes doing that kind of thing, does a good job of it, and has the time and energy to do it (and put up with flack, because there will always be complainers no matter how much one does).
There are also a couple of caveats:
It is important to keep focus when creating such a guide. It should be focussed on those creating themes or who will creating their own layouts for one-off sites.
For those who want fully realized ‘modern/post-modern’ sites a separate effort to create a theme and a (sub-)community around a theme designed to do that would be more appropriate than trying to shoehorn that onto the Hugo documentation.
Back to your question about a ‘colour scheme or background/image or logo’: those are CSS / HTML design questions not Hugo questions. There might be specific considerations on how to make that happen using Hugo’s Go Templating and/or Hugo Pipes and/or support for PostCSS but those specific examles are very theme-specific and are not generic questions to include in general Hugo documention, at least in my opinion.
It’s important not confuse the ‘site assembly’ tool with a theme and documentation effort that would look more like a ‘website IDE’ (or the CLI equivalent, at this point). It still seems to me you are conflating a whole environment build around ‘make’ and a C compiler like Visual Studio with ‘make’ and GCC.
Would a more developed environment built around Hugo be useful to quite a number of users? Probably, but it would require that those interested in making that happen work on it, or pay to get it done, rather than expecting Hugo to become that.
Probably more than just 2¢ of my thoughts on this.