Are "主题" and "内涵 interchangeable?" The sentence I'd like to say is:


  "Yes, it does involve the idea that the face of death brings out the best and the worst in people, yet other themes are involved."  


This translates to:


  “对，它蕴含着死亡会揭示人们的最好和最坏的方面的含义，但是本故事也有别的内涵。”


Can you use "主题" instead of "内涵"?
No.

主题 = 'main topic'

内涵 = 'meaning/connotation'

In English, the topic of a conversation is often very different from the meaning of the same conversation. The same applies to Chinese.