WordPress Theme Templates: Stylesheet Overview
Every WordPress theme includes a style.css file. A browser uses this file, commonly known as the stylesheet, to provide style to the website design, such as font types, colors, and sizes; graphics; icons; background colors; borders; and other styling elements.
The stylesheet targets areas of the site to style using CSS IDs and classes. CSS IDs and classes are simply a means of naming a particular element of the site. IDs are used for elements that appear only once on a page, whereas classes can be used as many times as you need. Although this file references style, it contains much more information about the theme.
At the very beginning of the style.css file, a comment block, or the stylesheet header, passes information about your theme to WordPress. Comments are code statements included only for programmers, developers, and any others who read the code. Computers tend to ignore comment statements entirely, but WordPress uses the stylesheet header to get information about your theme.
In CSS, comments always begin with a forward slash (/) followed by a star (*), and end with a star followed by a forward slash (*/).
If you make modifications to the stylesheet comments, the changes reflect in the WordPress Dashboard on the Themes page (choose Appearance→Themes).
Themes must provide this information, by way of comments, in the stylesheet header, and no two themes can have the same information. Two themes with the same name and details would conflict in the Manage Themes page. If you create your own theme based on another theme, make sure you change this information first.