I am not sure if I lost you. The Congo River Rapids is theamed with rock work, jungle, tribal... it is the other two water rides that are not themed.
The park started as a brewery tour for Busch beer, with Gardens for the visitors. They slowly added rides, games, and attractions without a central theme. Eventually it became a 'themepark' with a brewery walk-through. Then they knocked down the brewery to make room for Gwazi. So they have gotten better with themeing over the years, but the older stuff may be lacking in that department.
A few steps up from the average amusement park, but not quite on the level as Disney or Uni. If you take it for what it is, it is a good time.