If I understand what you're looking for, GTK. If it's a linux desktop, there's a super high chance that GTK is both installed and familiar to the user. However

If I understand what you're looking for, GTK. If it's a linux desktop, there's a super high chance that GTK is both installed and familiar to the user. However... > This would make vendors write more gui software for Linux What makes you think vendors want the native look and feel? Even on Windows, there's a long history of apps wanting to feel "unique". That's what they want: full customization to fit their branding.