regarding CSS 2.0 validation

Dear sir/madam

I have created a webpage consisting of an 'HTML 4.01 Transitional' source and 
two separate stylesheets, which are linked documents. One stylesheet controls 
the layout and the other stylesheet controls the font styles. 
       Firstly as I am interested in accessibility conformance is it 
recommended to use a stylesheet alone to control the layout? 
       Secondly how can I validate the entire website for CSS 2.0? When I 
validate a stylesheet it tells me 'To work as intended, your CSS style sheet 
needs a correct document parse tree. This means you should use valid HTML.," and 
then it gives me an error: URI: file://localhost/C:\WINDOWS\Desktop\Testing 
website (w3c)\stylesheet.css.. i don't understand quite what this means, any help 
would be much appreciated.

Regards, 
        Michael McDill.