Linux puskom-ProLiant-DL385-Gen10 5.4.0-150-generic #167~18.04.1-Ubuntu SMP Wed May 24 00:51:42 UTC 2023 x86_64

//usr/share/themes/Ambiance/gtk-3.0/apps/glade.css

 
GladeEditor GtkContainer {
    background-color: @bg_color
}

GladeWindow GtkGrid {
    background-color: @bg_color
}