I am using superfish menu module with drupal 7.7 and I can't figure out how to override the CSS in space.css or any other superfish stylesheet. If anyone knows how to, please share your input.
It supposed that the module has his own css, you need to edit those files but that is not a solution.
I recommend you to use some debugging tool like firebug and check what css you need to edit. Then write the css changes in your theme css file. With this logic if you update the module you will not loose all your custom changes.
I'd go with @byroncorrales answer and suggest that you use firebug or any web development tool to identify the styles that you need to modify. Also I believe that you have read the superfish module documentation and are adding the custom styles using the Extra CSS Class(es) and the Extra CSS. Apparently its merely a matter of CSS precedence and specificity that's giving you the trouble of not being able to override the CSS. You could refer this link to understand more on the issue and do some searching to identify and fix the issue as its been explained well in multiple places on multiple occassions.