CSS
From MozillaZine Knowledge Base
Cascading Style Sheets (CSS) is a language used to create stylesheets which describe the presentation of a document written in HTML or XML (including various XML languages like XHTML or SVG). In Gecko-based products such as Firefox, Thunderbird and the Mozilla Suite it is also used for styling the application's user interface. For example, themes make heavy use of CSS to change the appearance of the application.
CSS is a W3C specification.