numix-gtk-theme/gtk-3.0/apps/synaptic.css

/************
 * synaptic *
 ************/
GtkWindow > GtkVBox > .dock,
GtkWindow > GtkVBox > .dock > GtkHBox > GtkToolbar {
    padding: 4px;
    border-style: none;
    background-color: @toolbar_bg_color;
    background-image: none;
    color: @toolbar_fg_color;
}