To write the theme of a novel well, start by identifying a central concept that you're passionate about. Consider how this theme relates to universal human experiences and values. Build your characters and their conflicts around this theme to make it integral to the story. Also, use symbols, motifs, and recurring elements to reinforce the theme throughout the novel.
First, you need to have a clear idea of what message or emotion you want to convey through your novel. Then, think about the characters and their experiences that can bring out that theme. Also, make sure the plot and settings support the theme.
The key to writing the theme of a novel is to have a deep understanding of it yourself. Let it drive your creativity. Develop a plot that naturally emerges from the theme and ensures every aspect of the novel - from character development to the resolution - ties back to it. For example, if your theme is love and sacrifice, show characters making difficult choices for love and the consequences of those sacrifices.