What is the theme of misadventures of Max crumbly 1 locker hero

I would consider Imagination Vs Reality. Max has a great imagination. However, there are plenty of times when his imagination does not line up with reality, and Russell uses this to various effects.

Russell employs illustrations and crossed out text to show the reader that what was happening in the real world was different than what was happening in Max’s imagination. The illustrations often depict a fanciful version of his story. For example, he probably did not really try to give himself super powers with chemicals in his laboratory. However, it was drawn that way. This suggests that Max imagined it this way.