One of the themes in the novel is that ‘power corrupts those who possess it'. How does Orwell bring about this idea through the character of Napolean?

This is a pretty famous statement. It works in different stories too, like Macbeth. Basically, in animal Farm, once Napoleon tasted power, he needed more. Power can corrupt the soul and absolute power can push one into some sinister places. Once Napoleon had no competition and all animals were terrified of him, he indulged in his darkest desires. Orwell felt that without checks and balances people naturally gravitate towards corruption.