Themes are ideas that run all the way through a literary text. They are great to analyse because you can discuss the writer's intentions - what have they done? Why? What are they trying to make the reader think and feel? This allows your analysis to be extended and developed, allowing for you to write comprehensively about a given literary text.
In Dr Jekyll and Mr Hyde there are many themes you can analyse.
You will notice that some of these are opposites and these can be referred to as binary opposites or complete opposites. Examples of binary opposites are: hot and cold, wet and dry, light and dark.