How to save a .css file from TextEdit or Word?
First, use TextEdit, don't use Word.

Before you save your document, select Make Plain Text from the Format menu. (If the menu says Make Rich Text then you are already in plain text mode.)

If you save the document now, you should be able to give it whatever extension you want. Since the file is plain text, you can be sure that TextEdit won't add any additional formatting instructions that will turn it into an incorrectly-formatted CSS file.