The themes that can be selected to be used at this site are at various stages of being upgraded. The themes that come in the Tiki package are the farthest along, and are getting pretty close to being fully updated for Bootstrap 4. But some page elements such as menus and navbars are still being worked on so may look broken in some or all themes.
Other themes that will be available as aftermarket installs aren't as far along. The ones that are updated significantly can be selected, and the ones whose Bootstrap 3 stylesheets are still being used (they haven't been overwritten by a Bootstrap 4 SCSS compile yet) and look ok can be tried out here. Other themes aren't so far along and are more or less broken or otherwise not a good example of how the dev version looks and works. These themes aren't yet available to choose in the Switch Theme selector.
Probably all the themes at the site (those in the Tiki package and aftermarket themes that will be available at Zukathemes) will be ready by the time Tiki 19 is released in October.