If you had actually read the articles shared further up in this thread, you’d know that apps actually break: various widgets become hard to see in some places, or even invisible, and random artifacts can appear unexpectedly. This might not be what the stylesheet author intended, but it happens when their stylesheet clashes with whatever custom styling app developers have applied to some things in their apps.
The stylesheet is a part of the app’s code. If my app is depending on a library, I wouldn’t expect it to work properly if I swapped that library out with another one that’s just similar in scope. Likewise, stylesheets is something an app can (and very often does) depend on to work properly. Custom widgets are often written with the expectation that they’ll e.g. be displayed on a certain background color, which is provided by the stylesheet. If you swap out all of the CSS code in that stylesheet, that the app depends on, the widget might not be visible at all. The app becomes visually broken, even if its underlying app logic is still in place.
For a custom stylesheet to be guaranteed to work for every app, it would have to be tested for every app. I highly doubt that’s happening. Popular stylesheets already have app-specifix fixes in them, and as the app ecosystem grows this becomes more and more unrealistic to maintain.
Arbitrarily restyling apps is not a «functional workflow demand». If you have accessibility/readability needs, there’s light/dark mode, a built-in high contrast mode, a large text mode, and more. These are properly implemented and tested against.
More importantly, though, telling someone to stop developing software because it does not meet your particular requirements is a pointless argument. You’re free to find or develop software that suits your needs instead of assuming the intentions of free software developers and attempting to gaslight them into doing it for you. I’d be happy to discuss with you if it wasn’t for this, but now I think it would be more productive to lock this thread and move on instead. The original topic has been addressed in detail.