Tiki Bootstrap-based themes are exclusively visual skins separate from site functionality, for a pure design environment. Tiki layouts are highly configurable via multiple configuration options and template customization. Web designers working on Tiki don't need to think about anything but visual appearance, but do have to keep in mind that Tiki makes multiple visual options available to site admins, such as several basic page layouts, light and dark navbars, user interface icon sets, and so on, and Tiki themes need to support all of these. But it's easy to get up to speed by having a look at how existing themes are put together.
Please visit themes.tiki.org for more information.