@tio The system wide features should be implemented by your system. It’s not up to the app to support your window manager, it’s up to your window manager to add a title bar to apps that don’t use one.
It’s up to your global menu to detect menus from an app, there’s no API on Linux to do that, (there are hacks). It ‘s up to your theme to theme your apps, since there’s no API to theme apps on Linux, whatever the desktop you use. Pin this on the desktop or the distro, not the app.