With Bootstrap CSS framework now integrated into Tiki, adapting existing themes to Tiki is much easier. Bootstrap themes made for straight HTML (that is, themes such as those made with the Bootstrap customizer) will work for Tiki "out of the box".
Keep in mind that the role of themes on the Tiki platform is limited to visual styling and content placement and visibility, etc. Tiki themes don't have a functional capability in the way that WordPress themes do, for example. So naturally those themes converted for Tiki will retain the appearance, but won't have any functional ability beyond what Tiki provides.
Bootstrap themes for other CMS applications will work to some degree, depending on how compatible the targetted HTML is with Tiki's HTML. In other words, any styling and layout provided by the theme to style HTML element classes that Tiki doesn't have obviously will be missed. Therefore, some conversion will be needed to match the theme's CSS classes with those in Tiki's HTML.
This can be done either by changing the class names in the stylesheet or by changing Tiki's HTML. The latter isn't necessarily as daunting as it sounds. Tiki uses Smarty template files for layout, and a theme can have its own equivalent files that are modified to meet the needs of the theme. If the theme expects the main HTML page to have a certain arrangement of divs, for example, a layout_view.tpl file can be made to provide that arrangement. It functions as the bridge between Tiki's content display blocks and the theme's CSS rules.
Smaller page area component templates similarly can be made. There is some maintenance overhead to this approach because as the core files are developed over time, the theme-specific variants will need to be updated to stay compatible, but especially if they are made to provide the bridge for more than one theme, the effort would be worth it.
As of this writing, only some partial adaptations have been done internally to test the process. But a project is planned to work out the details of converting a WordPress theme, among others.