I assume not, but the emphasis here is that a new tool is homogenizing these projects and due to its scale it is more important that this homogenous output is up to a higher standard.

A hundred self-thought devs not implementing accessibility standards is a different problem than a school teaching 100 students lacking these standards in its curriculum.

Yes, because a majority just use a CSS framework to save time, and all the big/common frameworks have put a fair bit of effort into their default colours and typography.

I would rather go back to when all side projects used Bootstrap than this purple-on-purple-with-glowing-purple mess of stuff we have now.

The "default" light-mode look of most popular UI frameworks wouldn't have that same issue unless you put a lot of time into customizing your own styling, which most side projects wouldn't bother with (unless that look and feel was the point of the project). There certainly would be poor UI decisions but more likely in layout/placement/navigation, which could still be problematic for accessibility but probably not in a "is this color scheme even readable" kind of way.

Plus given time constraints, they generally wouldn't try to cram huge amounts of tiny text into every visible inch of the page without some intentional reason to do so (using that somewhat hard to read console-ish font Claude seems to love as a default).

Maybe the dark mode/terminal font/high text density look presents as "cool looking" at first glance for one-shotting evals so they've all converged on it. But to OP's point, this seems like a solvable (or at least mitigable) issue if models or harnesses were concerned about it.

It depends. As with everything else, those who know what they're doing and can make good looking and readable dark mode sites are the minority. Too many who think they're cool make squeaky white text on pitch black background and/or commit other sins. AI represents the average of the majority, not of the best.