To me, this is a big argument as to why devs should be making heavy use of parameterized values their UIs, as opposed to hardcoding things. An app making as much use of parameterized values as possible will not only remain decent looking and usable under most themes (except for themes that are badly built — nothing can help there), but also play much more nicely with accessibility settings like font size, font weight, colorblindness modes, etc.

I will caveat this by saying I haven’t worked with GTK and don’t know how well-tooled it is in this regard. If GTK doesn’t offer any/many parameterized values, then that’s on GTK, not app devs. They’re pretty well supported in macOS/AppKit, iOS/UIKit, and Android/Compose though and should be a cornerstone of any modern UI framework.