CSS on every page

I know there is a way to do this, but I am not sure how. Say I have 10 pages, I would like to have the the css code on each page, w/o having to put it on each page seperately. Do I just make a properties.css for instance then use php includes on each page header? What is the best way to do this. That way when I change the properties.css it changes it for each page.

Thanks