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.
Rename it and add the extension.