Option to replace/inherit the default background (paper) color

[Frank-Vog]

Great work, very nice design!

I use to pass a <div> to the library and there's no option to change (or inherit from parent) the default (#fff) paper color.

It would be elegant to inherit it from parent by default but having an explicit option would be great.

My goal is to have the justgage widget switch with the App theme (light/dark).

[robertsLando]

Would you like to submit a PR?