Consider what the topic light and dark refers to. they are contrasting matters, and we must find ways to represent this in the art produced. Maybe get creative by actually combining the two together. consider the types of materials you want to use, dont just limit yourself to pencil crayons, art is all about expressing yourself, coloured tissue paper seems like a great idea, maybe a montage?
You must also reflect on the type of scenario you want your art to be based on, relate the theme to a particular topic, such as life and death, where colour depicts life and death is represented much dull and darker in opposition. greater marks are gained when the art is made personal to you, perhaps you can use pictures of yourself or your surroundings as part of primary research.