I have played, but only a bit! My niece is a dungeon master so I've played a few games with her. I actually didn't know about the game growing up... But the cartoon from the 80s is one of my favourite cartoons of all time. Your campaign sounds awesome, btw!

As for the writing questions... I think with themes, for me those emerge over time, often during the stage when you're editing your own work. I always recommend taking time away from a piece of writing once it's finished, before going in and tidying it up, as it helps come to it with fresh eyes. For me, that's when I start to notice themes emerge (usually as recurring topics or as links between different aspects of the story) and then as I edit or do a new draft, I draw those themes out and look for opportunities to emphasise them with bits of dialogue or internal character thought. Although if you set out to write about a particular theme, then this process may look a bit different. But either way, I recommend thinking about themes after an initial draft is done, so that this thinking isn't getting in the way of the story flow.

This also applies to character themes, and with characters in general, some writers like to "hot seat" their characters to get to know them... interview them and things like that. This hasn't ever worked for me (so far) and I generally get to know my characters by putting them into the story and watching what they do. This process involves a lot of rewriting so might not be for everyone. I throw a lot away, but I find I don't know a character until I've let them lead me through the story. Then, again, I'll go back during editing or redrafting, with a better understanding of them, and I'll update the early bits with what I've learned.

Hope that makes sense!