There are many themes in Dr. Jekyll and Mr. Hyde. Among some of the more prevalent is the theme of good versus evil. The good vs evil shows up in Mr. Hyde's evilness toward others and society in general, whereas Dr. Jekyll is the polar opposite. There is also the internal struggle of good versus evil.
Another theme is that of repression. Victorian era people were bound by terribly strict rules of conduct and tradition. Everything was to remain, on the surface, calm, sober and dignified. Violence, outbursts of emotion, sexual proclivities, all happened behind closed doors and were not for public display or discussion. The problem is, with all of that repression comes Mr. Hyde, whose tastes and morality don't figure in to the Victorian ideas at all. He's 'free'.