GNOME White Title Bar
Vivaldi has broken free of the 'GNOME White Title Bar' in Fedora 25
How can I get Firefox, Thunderbird and more apps do the same?
Vivaldi just undecorates itself, Steam does the same thing, and I think Chrome does by default, too. But it's not something you can normally achieve with Mutter, as it has no built-in functionality to allow the user to remove window decorations. You can improve the visual appearance of GNOME via themes, which you can get from gnome-look.org, or you can try installing the Maximus extension for GNOME, which hides titlebars on maximized windows.