VuFind is developed using templates for HTML and CSS for design.
The layout is controlled by the Yahoo UI CSS which means that by modifying the class declarations within the HTML, you can modify the layout. For more information on how to use the Yahoo UI CSS, please see the Yahoo UI web site. To modify the colors and design elements, you can edit the styles.css file located in the web/css directory.
All of the HTML is stored in Templates. There is one main template that controls the overall web site - the layout.tpl file stored in the web/interface/templates directory. Each module also has it's own templates directory where the individual templates for each web page are stored. They can be modified as needed.