I need to create a .css file on my Mac. I created it with TextEdit. How do I now save it with a .css extension? It seems that very single File Format it offers will give it a different extension (rtf, html, odf, docx, xml, doc)?!

Safari Version 8.0 (10600.1.25) On a Mac OS X Yosemite Version 10.10 iMac 27-inch

I need to create a .css file on my Mac. I created it with TextEdit. How do I now save it with a .css extension? It seems that very single File Format it offers will give it a different extension (rtf, html, odf, docx, xml, doc)?!