If you want to join multiple sheets, it is as simple as copying all the text in the files to another file, though there might be some curve balls thrown along your way. If the multiple sheets have elements with the same name, you should see what properties they have and copy all the properties over to only one element so that there are no duplicates. Besides that, unless you have some other problems you might want to share with joining the sheets, it's not such a hard task to do it.
This is an old thread, but if you are still looking for a solution, you may try out the site below. Listing it here so it may help others who need the tool. http://www.cleancss.com/ Simply paste the all the css you want combined in the box and it will clean up and merge selector with the same name together.