That's how you end up in a situation where the contents of the window clash with the dressing because more and more apps can't be themed. If they want a consistent look they'll need to fork or write a whole new set of apps.
For some apps I don't care too much, because I run them in full screen anyway, and their specific look is adjusted to their function: IDEs, DAWs, even graphic editors. But if they do support DE-wide themes, I do appreciate that!
If you’re thinking of the website I’m thinking of [1], the screenshot they lead with isn’t just inconsistent. It’s broken, with light text on a white background. And I’ve seen apps fail in that same way for real when trying custom GTK themes.
It’s the same reason web browsers can’t change the system CSS stylesheet without extreme measures like Reader Mode that strip almost all of the design work out of the site: it’s hard to mix design work from two different people (the application author and the theme author) without risking a total mess.
GNU/Linux apps will never have something like OLE 2.0 or XPC that actually works consistenly across desktops.
There is D-BUS, but not everyone cares it is there.
Do any Linux experts out there feel the same way?