The theme is the thread running through your novel – the thread can give its coherence.
The theme is what your novel is about, whereas the plot is how the theme is presented through the characters and their actions.
The essence of the theme is more often than not, conflict. The conflict has to be solved by the end of your novel, one way or another. The problem itself can be as diverse as you like – lets face it – sometimes human behaviour can truly be stranger than fiction.
In crime fiction, the theme can revolve around intimate family secrets, or the dissection of the banal life of the psychopath or murderer.

In a crime novel, it is the crime itself that is the specific event to catapult the reader into the action – whether it be in the time leading up to the crime, or in the aftermath.
it is the character’s reactions, such as self-loathing or introspection that your readers will enjoy. There is a lot to be gained from developing the human interest in your novel.

One of the most common themes is that of self-development within the protagonist. The reader likes to follow the development of strength to overcome weaknesses. For example, both detectives in my current WIP have tainted backgrounds – for very different reasons. They have to cope with their own issues whilst trying to find the killer of three children. Flawed characters often seem more realistic – adding depth to the theme.

Happy Word Flow One & All