Themes
A theme is a plugin. A theme plugin overrides parts or the entire default theme that is built into Elgg. All themes override elements of Elgg's CSS and some themes additionally override views used to create a site's HTML. A theme may also include graphics or JavaScript code.
Elgg has its own CSS framework that uses the view system. The primary CSS view is 'css/elgg'
and it includes the various modules for layouts, navigation, forms, and other style categories. Chapter 9 describes the CSS framework and provides a tutorial for creating a theme.
Code location
CSS views: /views/default/css/