If it's not in the Trash then you may be able to extract the data from the browser's cache file. If using Safari, this can be found in [user account]/Library/Caches/com.apple.Safari/Cache.db
Use a text editor such as the free TextWrangler to open the cache file. How long this will take, or if it can be opened at all, depends on the size of the file and your system (a 100MB cache file opens here in around 8 seconds.).
With the file open in the editor, set it to soft wrap the text for easier reading and then search for some CSS that you know the stylesheet contains. If/when you find the code, copy it and paste into a new text file.