I use legacy file manager myself i dont like the file manager, it has some extra features but i like the legacy format better.   Just go to your css file and click edit file, not code editor but just edit file...

also i turn off the file check at the top of your legacy page.. youll see settings, thats just me..

just make sure you are not in compat mode or you will get a really funky display.

alot of people frown on editing files on the server, but i have never had an issue with it and it saves me time.