There are many themes in Mrs. Frisby. For example...
1. Courage, especially in those who are small, can overcome great obstacles.
2. Loyalty to family and friends is important and valued.
3. Teamwork is valued and successful.
4. Surviving on one's own hard work is better than relying on others (such as stealing from humans).
5. Everyone is an important part of a team, no matter how small or seemingly unimportant.
6. Technology has its place in our world and should be used carefully.
7. Perseverance brings reward or relief.
8. Life is uncertain.
One central theme to Mrs. Frisby and the Rats of NIMH is the importance of family. Mrs. Frisby's main reason for taking any action in the story is her love and devotion to Timothy. He's sick and/or weakened from his sickness; therefore, he can't be moved. The importance of staying with Timothy is what gives her the courage to talk to Mr. Ages, seek out the owl, and eventually go to the rats of NIMH.
"I'm Timothy's mother. If you, and Arthur, and others in your group can take risks to save him, surely I can too."
Family is important to Mrs. Frisby, and her devotion to it gives her bravery and courage. That's a second theme of the story. Mrs. Frisby is willing to put herself at risk in order to save Timothy. That's both brave and selfless.
Mrs. Frisby was afraid.
Then she thought of Timothy.... She told herself she had no choice. If there is a chance the owl might be able to help me, I must go.
A third theme of the story is societal belonging. The rats of NIMH are brilliant. That makes them awesome to readers; however, it's important to realize that their advanced knowledge also makes the rats quite dissatisfied. Yes, they are very intelligent, but they are also intelligent enough to realize that they don't fit in anymore. They are rat outcasts.
"And yet all was not well. After the first burst of energy, the moving in of the machines, the digging of tunnels and rooms—after that was done, a feeling of discontent settled upon us like some creeping disease."
The rats are not content with their current living situation. They are not content with their place. They are not content with their current way of life. Simply put, they are not content with being "normal" rats. That's what motivates them to come up with "the Plan" and start their very own society in the mountains.
There are many themes in the book. A few good ones could be: perserverence, bravery, and teamwork.