As of today, Mutter will style legacy titlebars (i.e. of X11 / Xwayland apps that don’t use client-side decorations) using Adwaita on GNOME.

Shadows match the Adwaita style as well, including shadows of unfocused windows. These titlebars continue to follow the system dark and light mode, even when apps don’t.

GitLab Merge Request: frames: Load libadwaita on GNOME