The quickest way to overhaul the look of a forum is to change its colors, fonts, and other visual (but non-graphical) decorations. When we looked at the basic elements of a style earlier, we learned that a theme defines and controls these concepts through the formatting rules of a Cascading Style Sheet (CSS). CSS formatting rules add stylistic data, such as specifications for fonts, colors, sizes, borders, and spacing to a document structured with a markup language like HTML. Since phpBB creates documents of this kind to display a forum, the preferred method for changing these parts of the outward appearance of the forum is CSS.

CSS rules can be stored in three different places: in an external CSS file such as subBook.css, in overall_header.tpl, or in a combination of the overall_header.tpl method with some database storage. subSilver uses the combination method, and therefore subBook will also do the same.