(from http://doc.tikiwiki.org/tiki-index.php?page=Styles and Themes - to be updated)
The presentation of elements, the visual style of your theme, is created using CSS (Cascading Style Sheets).
W3C provides tutorials and other tools for learning CSS principles: www.w3.org/Style/CSS. See also the CSS links in the Directory.
You can either start with an existing theme style sheet and copy, rename and modify it, or you can start with a "blank" CSS file and create the new style from scratch. To start from an existing CSS file, see Modifying an Existing Style Sheet; to start from a blank CSS file, see Starting Fresh — CSS.