Gtk+ 3.22 theme support

Just a quick thought. I'm running gtk3-3.​22.​0-2.​fc25.​x86_64. I just run Sugar, a heavy user of the Gtk+ theming system. Usually, now is when I submit a patch to port over some of the changes to the themes.

This cycle, the Gtk+ contributors had been saying that the theme api was made stable in gtk 3.20. And hell yeah - they are right. I was thinking of putting some pictures to show you just how it is exactly the same - they got perfect compatibility! But showing pictures would be a waste of bandwidth!

So just to recap - Gtk+ 3.22 is a great toolkit. Beautiful api. Wayland, X, Broadway and Win32. Idiomatic python, c, c++, javascript (somebody even posted a JSX/Gtk+ example) and of course Vala. Best of all you can just use CSS to change how everything looks.