grow your wiki
Wikis that use a wide variety of user interfaces and navigation can confuse visitors, who may give up looking for information on the wiki. The ability to choose a custom theme for each space (section) of the wiki may encourage creativity and unique branding, but visitors to the wiki will need to re-learn how to navigate each unique section.
Users will stop using the standard navigation functions in the wiki and revert to the search function all the time.
Encourage space/section administrators to use a consistent theme for their section of the wiki. Colour scheme differences are okay, but inconsistent navigation should be avoided.