This theme is based on some sketches I made two (three?) years ago. Back then I wanted to write a whole window manager, but I never started. Xfwm cannot support the behavior I had in mind (with the resize buttons maximizing on right click and similarly hide and close combined in one button), but at least I can have the look I like (with one button too many, but I don't mind that).
If you really need a maximize button, don't use this theme. My preferred maximize button is called "alt f5" ;) .
Those "diagonal things" are the hide and close buttons.