The developers of Qt applications do not force how the UI will look like. Users can customize it with local style sheets (usually from the desktop environment).
You can do that under Gtk3 too, but like with Qt5 it only allows small adjustments in line with how the developers built the UI. You can affect layout and style of certain components, but that's it.
From the perspective of making design decisions, it's somewhat equivalent to swapping the phone shell on a Nokia 3310.
GTK also supports customization via local style sheets.
In Qt it's absolutely possible for applications to force client-side decorations and force a certain stylesheet. Maybe it's not common in the apps you use, but again, it's up to the app developer if they want to make use of those things or not.
That's just plain FUD. The only release I remember breaking CSS was 3.20, and that was because all the CSS was refactored to allow for way more theming possibilities.
Regardless of that I don't see anything based on "cascading styles" as lending itself to a stable theming system anyway. The CSS is probably going to change any time a widget is added/refactored/bugfixed, this holds true on the web as well once you build up a complex library of React components or whatever. The point of it is that there are multiple styles from multiple sources that can cascade together. It's powerful but it can result in a lot of complexity, anyone who's had to add !important directives can attest to that. It needs to be strictly managed by the developers to really work correctly and to prevent the style overrides from getting out of hand. Allowing custom user CSS is only for power users who understand the caveats.
