Yes, the css works for all the top-level directories. The folder layout is like
Website
-css (Folder)
-img (Folder)
-subpage1 (Folder)
-product1.html
-product2.html
-subpage2 (Folder)
-product3.html
-product4.html
-index.html
-about.html
-subpage1.html
-subpage2.html
The css works for pages "index", "about", "subpage1", and "subpage2". It does not work for those in folders "subpage1" or "subpage2" (i.e. product1, 2, 3, or 4).