I’m sorry if this isn’t what you were asking, but the software used to create CSS files is dependant on your operating system.
A few common ones for windows: (based on personal experience)
Notepad++
(Free!)
Dreamweaver
(from adobe)
Webuilder
(Currently using this one myself)
A couple for OSX: (based on friends / tutorials / articles)
Coda
TextMate
Hope that answered your question, if not I have seen a ton of these same threads on here, just do a search and you should get a bunch of helpful information.