Except on OSX, what you want can't be done because all those widgets are drawn on real windows which are rectangular (unless you use some low-level tricks to chop holes out of the window concerned; not an approach I recommend even though I've got code – for Tcl/Tk, not Python/Tkinter admittedly – which does it). OSX is the exception because there widgets (mostly) aren't drawn on real windows, but rather use a lighter-weight system (because that's how OSX works).
You can tune the widgets to have less of a border than normal, which is OK if you're using a style that keeps them fundamentally square. Or if you're using a canvas for your “interesting” surface, you can build button-like things on the canvas which may well be better for what you're actually doing.
To change the background color of a Ttk frame, you'll have to apply a style to it. The best instructions on style creation I've seen so far are in the tkdocs style tutorial, which is where I go when I want to look up how to do these things. (Yes, the Ttk documentation ought to cover this itself, but it doesn't yet.)