Created attachment 318058 [details]
Screenshot of the issue (toolbar is usually dark, but the separators are light though should be invisible)

If I style my separators with a background-color (same works with border-top):

    separator {                  
        background-color: @button_grey;  /* light grey */
    }

And then I have an invisible separator toolitem:

    separator = Gtk.SeparatorToolItem()                                     
    separator.props.draw = False

It still gets drawn, see the attachment.

Further more, there is no extra style class or anything that I could use to select the invisible ones and make them invisible from css.