Changing the color of default frame border

[Mudcat]

Hello,

I would like to change the color of the default border around frames or
widgets. Note: not the highlight border, but the default border. There
have been many previous requests about the highlight border, but I
couldn't find anything on changing the default border color.

I know how to change the border relief and width but have found no way
to change the color of it. Is there a way to do that?

Thanks,
Marc