Currently, the only way to theme GDM is by compiling GNOME Shell with custom colors–that’s what we did with gnome-shell-maia.
manjaro-gdm-branding is just that–the Manjaro branding. We used to have manjaro-gdm-theme that changed the color.
Replace gnome-shell-maia with the regular gnome-shell package:
sudo pacman -Syu gnome-shell
Also change the Accent Color in the Custom Accent Colors GNOME Shell Extension preferences.
You will need to log off or reboot for the change to take affect.