While true for things like firefox, and GUI libs that think they know better than the host, its not ideal, and entirely dependent on the language/toolkit being used. And it's a side effect of X being too low level and not having a standard widget toolkit as one gets on Windows/mac. On those platforms, it's not just the widgets but file open dialogs provided by the OS, skinned consistently across applications, and things like global keyboard shortcuts (ex, copy/paste) tend to work close to 100% of the time.

If one goes back to the win3.x/macos7/etc timeframe it was almost unheard of for applications not to follow system wide color themes. WHich is why "dark mode" 1995 worked more reliably than it does on any platforms today. Today, doing something like inverting to a "dark" theme is suddenly something that each application hacks in, and when the OS provides a theme option, the applications frequently don't follow it correctly/etc.