I don't think you'll be able to change the appearance of Dashboard widgets. Those aren't icons, they are tiny User Interfaces - similar to very little programs.
EDIT - didn't read your second post, yeah, that can be done somehow to the black icon. Don't know why you'd do it though, I only ever see it for a split second before Dashboard installs it and tucks it away in the Library somewhere.
I'd steer clear of doing too much customisation, you can end up with something that will make your whole system act weird if you aren't careful.