CSS (Cascading Style Sheets) is a programming language that defines the format and layout of a Web page. For example, Web designers use CSS to create columns and boxes on Web pages. They also use CSS to style elements and define the colors, size, padding, etc. for everything from footers to text.
CSS applies styles with classes. A Web designer first creates and defines a class, then applies that class to all of the images, paragraphs, etc. he or she wants to style in that way. For example, one could define a “page-title” class that styles text as 24pt, navy blue, and bold, then apply that class to each page title on the site.
Developers create separate .css stylesheet files and place them in their own directory, rather than embedding the class definitions in the site’s HTML Web pages. Separating a website’s style from its content makes it easy to tweak the style once in a single place. The changes will then “cascade” to all the pages that reference the stylesheet.
WordPress administrators can use CSS to customize themes. The style.css stylesheet appears under Appearance | Editor in the Administration Area.