The presentation of elements, the visual style of your theme, is created using CSS (Cascading Style Sheets).
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.
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.
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.