How does a genre affect the theme of a story - like A Christmas Carol?
This site will give you a definition, plus types of literary genre.
http://www.sevenoaks.wa.edu.au/linkpage/reading/read4.html
If "A Christmas Carol" were meant to be humorous, how might the author have changed his ending. What if the story were to be a mystery? or an adventure story? If you think in these terms, you can see how the theme might be changed.