Mate MenuBar Help

Are there any contributors/developers who work on the Mate MenuBar Applet? If there is perhaps one of them could tell me why that the menus Application, Places and System have their backgrounds first painted with bg_color and then finally painted with dark_bg_color?

I’ve been playing around trying to make a custom theme in gtk-main.css and when I set dark_bg_color to black with an alpha of 0, to make the panel transparent. I thought the menus would also be transparent but to my surprise they now are the colour of bg_color.

Which means the menu windows are first being painted with bg_color, followed by dark_bg_color, and to me seems unnecessary to render the menus. Unless of course there is a reason for doing so.