What are two major themes in the book and how does the book present them?As you can see, this are two questions but they go along hand and hand so...

A major theme in Mrs. Frisby and the Rats of NIMH is
freedom. O'Brien explores this topic by telling a tale that might acquire an allegorical
dimension. Thus, the people from the lab would represent the oppressors (those who take
away freedom) and the rats the oppressed (those deprived of freedom). An oppressor
normally acts out of pre conceived ideas that the oppressed are inferior and therefore
they must be manipulated and subdued. This is what happens to the rats of NIMH: the
people from the lab capture them and use them to make their experiments. The rats'
capacity to acquire knowledge equaling that of human beings might imply the message that
the people who are deprived of their freedom through oppression are also human beings
capable of acquiring all the knowledge that the oppressors
possess.

Another major theme in the novel is courage. The
author presents courage as embodied in the character of Mrs. Frisby: she risks
everything (even her own life) in order to save her family. Mrs. Frisby goes through
different situtations in which she has to show an ever increasing amount of courage:
first by going away from home at dusk and running the risk of being captured by the cat,
then by flying on Jeremy's back when she is terrified of heights, and ultimately, by
offering herself to put the sleeping powder in Dragon's food. In this way, we can see
how her experiences led her to gradually conquer fear. Her victories over fearsome
situations enabled her to accomplish those actions required to save those she
loved.  This priorization of others over her own life transforms her into a
heroine.