Directory layout

I have a completed site that works fine. But I want to rename all of the pages to an index page so the url shows “www.website.com/Aboutus” instead of "www.website.com/aboutup.php. How do I link back to the style.css page if its in its own folder?
this is how i currently link to my css.

<link href="css/style.css" rel="stylesheet" type="text/css">

When I change aboutus.php to index.php and move to folder aboutus the css is not found. Do i have to have a css folder in every subfolder? and same for all my include files ex. header.php, nav.html, and footer.html?

As regards the stylesheet simple change the href atrribute to /css/style.css the slash at the beginning says that the css directory is a subdirectory of the root directory rather than the current one.

If you are using a Unix system remember it is case sensitive, that is Aboutus is not the same as aboutus.

I would normally say use url rewriting to achieve what you wish but you subdirectory approach should achieve what you want.