OroThemeBundle provides the basic theme support with the ability to change the original UI applying the extra CSS/SCSS files that are grouped into themes.
The bundle can provide CSS and templates that are required for its UI
UI of bundles (css and templates markup) should be “themable”, it means that CSS and UI components of bundle should be affectable by theme to change look and feel of application (at least colors and icons)
A theme is a set of CSS/SCSS files that will be included at the end of all CSS files to cascade styles and change look and feel
For more information, see the topic on back-office themes in the developer guide.