> How I can make the toplevel window also take the theme 'clam'?
Short answer: you can't. No directly anyway.
Long answer: As you might be aware, there are 2 widget sets in tk/tkinter, the "old" one for which classes are directly in the tkinter module, and the new one that are in the ttk submodule. Only the second set supports theming, not the first one. Unfortunately, there are a few widgets that exist only in the first set, and Toplevel is one of those. So no theming is directly available for toplevels, and you can change whatever you want via style.theme_use, it won't be reflected on toplevels.
By the way, as you wrote the code above, it won't be reflected on your button either, since you used the Button class, which is taken in tkinter directly, so it is the "old" Button class, not the new one. To get the new one, use ttk.Button, not Button.
For your toplevel, there is however a simple workaround: Since there is a Frame widget in the new widget set, you can simply insert such a frame in your toplevel, make sure it will take the whole space, and then insert your widgets in this frame rather than in the toplevel directly. The code for your 'window' function would then become:
(Note also that I have put the creation of the button and its packing in 2 lines. You should never do variable = widget.pack(…) since pack does not return the widget. It always returns None, so doing so won't put your widget in the variable).