Validating my Stylesheet

I ran the CSS1 validation on my stylesheet and it came back with "no error or 
warning found." However, it then says "To work as intended, your CSS style 
sheet needs a correct document parse tree. This means you should use valid 
HTML."

Unfortunately, I don't know what that means or how to adjust my stylesheet so 
that I can use the W3C CSS logo that says I conform.

When I got "no errors or warnings" on the W3C HTML validator, it told me I 
could use the logo. With the CSS validator, it does not tell me I can use the 
logo.

Can you help me?