A theme can be configured to dynamically respond to changes in the content, content types, user input, user role etc. A theme can also be used to override (modify or replace) the text and variables that are generated by modules. For example, a theme can be used to replace the default label on the Search button or to hide and display certain fields that are part of a particular content type. You can also override the default CSS classes that might appear in your content. Often a theme is used simply to define the look and feel of an entire site, but it can also be used to customize the look and feel of certain sections of a site, of certain types of content, and even of individual nodes or pages. For example, your theme could specify a different look for the front page of your site. Depending on the kind of theming work you are doing, you will need some knowledge of the following subjects: