The counter argument to that is "so let the user theme the app, to suit their own desktop", which would be a decent solution, but:
1. My vision for my app might conflict with your vision for your desktop. Maybe I want this button to be a light blue because it meshes well with some other elements in the app, but you want it to be a darker blue because it fits with your desktop's color scheme. What happens then?
2. This still doesn't guarantee that the app will look good. If you theme my app's home page, but don't theme the rest of the pages, then sure it'll look good at the home page - but as soon as you start using it, the look will fall apart. Or, what if I push an update to my app which adds a new page with a new kind of UI element? Do you really want to be maintaining your desktop theme for every single app you have?
3. This adds a burden on me as the developer to make parts customizable. This is the least convincing argument in this list IMO, since if there was better tooling and infrastructure for theming in GTK this wouldn't be a problem - but there isn't, so it is still a problem.
As a practical example, my app makes use of a WebViewGTK to display some info. I inject some custom CSS into this web view to make it look like Adwaita. This touches on points 2 and 3:
2. The webview has some UI widgets which aren't present in the rest of GTK, like a sticky header bar. You would have to manually maintain a stylesheet for this single element.
3. I now need to write a way to let users theme the custom CSS inside the webview, rather than just the CSS of the GTK widgets themselves. (I have already written this, but it's still a maintenance burden.)
