stylesheet option in Page Settings?

I’d like to use a different stylesheet for my ‘about’ page. When I go to edit the page, in the Page Settings there’s an input box labeled Stylesheet. I’ve uploaded a new css file in the same folder as my main style.css called style_bio.css. How do I attach it to my ‘about’ page? I typed the name of the new css file (style_bio.css) into the stylesheet input box, but it didn’t work.