Pamac-gtk does not match Xfce theme

Once installed it will use the libadvaita style and color-scheme and will ignore the changes in System Settings > Appearance. It can be done to follow the KDE style and color scheme by doing one of this things discussed here for other GTK4 applications:

You will still get this Adwaita warning if you run: GTK_THEME=Breeze pamac-manager

Using GtkSettings:gtk-application-prefer-dark-theme with libadwaita is unsupported. Please use AdwStyleManager:color-scheme instead.