Customising Tk widgets

[Peter]

I am using Windoze, I suspect the appearance attributes I am asking
about here are platform dependent?

Using Tkinter, I would like to generate a Checkbutton that is filled
in with a solid colour rather than a tick mark when selected.

Could somebody provide some pointers as to how I could achieve this?

Also, John Shipman's Tkinter reference shows the Radiobutton drawn as
a diamond and yet when I create one in Windows I get a circle - again,
how and where do I need to look to change this behaviour?

Thanks
Peter