Please see Marketplace Themes for additional themes that can be used with Tiki. This feature of the site is just getting under way (for themes compatible with Tiki 15 and newer) and a few dozen themes are in the pipeline from one source at least.
Be aware that any bootstrap.css style sheet can be used with Tiki, though its coverage of all features may not be complete, regarding some details. This style sheet can be produced from one of the online Bootstrap style sheet generators, or coded by hand.
Also, people looking for or needing custom themes can go to https://tiki.org/Consultants for more information and to request services.