One main theme could be the power of imagination. Since the moon is something that is often idealized and can be depicted in countless ways, it allows for a great display of the character's imagination in the story.
Another theme might be the pursuit of beauty. The moon is widely regarded as a beautiful celestial object, and the act of drawing it in the story could be a way for the characters to pursue and capture that beauty. They may be on a journey to find the perfect representation of the moon's beauty through their art.
The theme of self - discovery could also be present. As the characters engage in drawing the moon, they might learn more about themselves. For example, they could discover their patience, their creative abilities, or their connection to the natural world. Through the process of trying to draw the moon, they may unearth hidden aspects of their personality.