You can display a block without color by using a <pre> block.
And I think StackOverflow is using Google Code Prettify, but I haven't figured out how to get it to show CSS yet (though it claims it should, attaching class="prettyprint lang-css" in a pre tag doesn't seem to work.).
CORRECTION: Oops, apparently StackOverflow uses Google Code Prettify not syntaxhighlighter as was previously posted, thx for the correction shog9, it's always nice to have the right information. Despite the correction, the class="prettyprint lang-css" attribute still doesn't seem to work, even with supported languages such as C.