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)?!
It sounds like you are in rich text mode, so the editor is trying to save the file in non-plain text format. In general for css/html/code you want just plain text.
In the "Format" menu, choose "Make Plain Text" then when you do a "Save As..." you should be able to just type whatever extension you need.
EDIT -- As Tetsujin mentions, you do should uncheck the 'if no extension, set as .txt' in the save menu so it doesn't try to append .txt as an extension.