Presentation
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.
Tiki methodologies
You can start with an existing theme style sheet and copy, rename and modify it or you can start with an empty CSS file and create the new style from scratch.
New in layout.css and design.css
In Tiki 3, a layout.css file was introduced that provides basic layout selectors and properties and some default colors (and colors meant to be overridden by the theme stylesheet), which is imported by the theme stylesheet. In Tiki 4, this was split into a new layout.css file, for layout and positioning rules, and a new design.css, for some default design rules.
To start from an existing CSS file, see Modifying an Existing Style Sheet; to start from a blank CSS file, see Starting Fresh — CSS.
Be aware that Tiki also has CSS files for specific features, such as the calendar, and CSS files that are part of external code such as jQuery.