Of course, utilisation of multiple themes is the best way to make detailed, realistic builds. Limiting yourself to single themes in my opinion is pretty boring and makes your zoo areas over themed which is kind of what one would expect with how frontier markets these scenery sets. There are several themes which serve as a basis for building materials such as Africa packs plaster walls, aquatic packs stained wood walls, tropical packs brick walls, twilight packs shingle roofs, South america / tropical / soon to be oceania packs thatch roofs. The base game has tons of themes that can be used as well, mixing Africa packs and conservation packs scenery with the classic theme works well for a modernisation of a classic building.
There's endless options for mix n' matching scenery as long as you are creative enough to think how 2 different pieces can work well together. Bonus is doing this allows you to get familiar with the scenery catalogue which expands your knowledge and ability to do impressive builds.