You can overwrite a CSS class by making it "more specific".
You go up the HTML tree, and specify parent elements:
div.sidebar { ... }
is more specific than .sidebar { ... }
You can go all the way up to BODY if you need to:
body .sidebar { ... }
will override virtually anything.
See this handy guide: http://css-tricks.com/855-specifics-on-css-specificity/