The theme of the book the Shadow Thieves is overcoming adversity for the sake of others. In the story, Philoncron crates an army using the shadows of children. When the children's shadows are stolen, they are left completely drained of energy. When Charlotte and Zee realize what's happening to their friends, they gather the courage to go to the Underworld and confront Philonecron. They managed to defeat Phil and save their friends, demonstrating their ability to overcome adversity despite all odds and prevail against that which opposes them.
Another example of overcoming adersity in the story is when Charlotte rescues Zee when he starts sleepwalking on the dangerous streets. She must get over her cowardice to exit the safety of her home and save her cousin from death-or worse.
One more example is when Mew fought off one of the Footmen. The Footman was trying to steal Charlotte's shadow, but Mew, the small and young kitten, bravely struggled against the monstrous, hideous, powerful Footman to save Charlotte and Zee.