I would not say that the desire for power is a central theme of the book, Of Mice and Men. For example, the word power is not even used once in the novel. However, it is there in the background in two ways.
First, the story begs the question of why Lennie and George are in their situation. They have no money, no place to live, few prospects of success, and they are moving from place to place just to survive. If we think about it, they are there because they are poor owing to the economy. Banks have taken away people's lands, creating great hardships for the common working man. In this sense, wealthy people are taking advantage of the situation to create a class of people who are perpetually poor. We can say that they are seeking after power, and it creates a dangerous society where desperation rules.
Second, in the context of the book, we can also see power dynamics. Lennie and George always have to walk on eggshells, because they have no power. And what makes the situation worse is that Curly, the son of the owner of the farm, is antagonistic. He want to exercise his power. So, he makes it hard for Lennie in particular. In the end, after Lennie accidentally kills Curly's wife, Curly and his men, because they have power, seek to lynch Lennie. Those who have power call the shots. Therefore, it is always dangerous for those without power.
In Of Mice and Men there isn't so much as a desire of power but more a struggle to live day to day. Remember this story was written around the time of the great depression and people were just trying to make a living and reconstruct their lives by readjusting to poverty. The same applies to the main characters Lennie and George who are just looking for work until they are able to save enough and get their own place. The only person in the novel who really seemed to have an obsession with control (or you can even say power) is Curly. His wife is unhappy and his ego and hand hurt because of his controlling attitude.