One plugin could seemingly manipulate all styles to suit, you can define differences there as well.
Edit: Do not turn on debug mode and edit your master style, no!
Thanks for the link and info. It's not simply just the CSS I want to change, it's the template I want to change too. I have edited the Post Thanks box template and renamed certain <DIV> classes and added new div's also. So I want to make a plugin OR addon which will overwrite the default code/template.