After a couple of comments on the last post, it seems to me that not everyone is entirely comfortable with the idea of working themes into novels. Certainly, it’s something that can go wrong, resulting in something that is heavy handed and didactic if you aren’t careful, but it’s worth trying, because when it goes well, it really adds something to a novel. I know Court of Dreams became a lot better in a relatively short space of time when I sat down and really analysed what the novel was about.
There are two distinct times you can add in themes. The ideal way is at the start, so that you build up everything with your core ideas in mind. That creates a very coherent novel, but is also the way where you have to be careful to develop everything fully if you want results that are broad enough.
The other way is to add them in half way through. What you have to recognise here, though, is that you aren’t actually adding anything. You are simply recognising what is already there, and trying to bring it out slightly more. If you’re writing epic fantasy, for example, you might want to focus on themes of family or duty, political expedience or trying to do what’s right.
Having a core theme doesn’t mean that you have to abandon all other issues. Indeed, you shouldn’t. It does, however, mean that one tends to take precedence, so as to avoid an unfocussed effort in which there are several distinct plots pulling it apart (as opposed to several interweaving strands all pulling in the same general direction)
Once you’ve decided on a theme, then it’s simply a case of adding it in at every level you can find. Have characters face large and small dilemmas related to it. Have a world that reflects it in some fundamental way (and which might change as a result of the story). Have subplots that reflect different takes on the same core concept. Include symbolic moments, items, and actions related to it. Think of all the stuff you were asked to unpack when discussing classic novels at school (discuss the way X approached the theme of Y). Now simply do the reverse, for a result that suddenly has much more depth.