Ideology is more powerful than action. When someone is oppressed, wants change, or needs change, then an ideology has begun. If others agree with it, it can become a movement. And from there, no action can stop said movement.
Real world example: Colonial America. The Colonials were sick and tired of England's overreach and taxation. The ideology was more powerful than the actions of the king and they succeeded in creating a new country, which is now the United States. That ideology of freedom is still present today in the US. This relates not only to the theme but the Book Thief as well because the people were oppressed by the British monarchy and fought back in seeking of freedom.
In the book thief, the setting is in Nazi Germany where many people were discriminated due to their race,disability, amongst other factors and were hunted down and killed for it. The people never gave up on their beliefs and religion, even though they were being prosecuted for it.
A jewish shop being boycotted simply due to the race/religion of the shop owners. Said owners were eventually likely taken to a camp and killed by Hitler's army.
The Book Thief shows a perfect example of this because one of the characters possessed a book that Nazi Germany did not want them to have. The ideology is that some people chose to break the law to have freedom and try and escape the oppression. In this case, the theme fits because no matter the action someone took, the ideology of the people yearning for freedom of information was much greater and helped push them through hard times.
"Sometimes I imagined how everything looked above those clouds, knowing without question that the sun was blond, and the endless atmosphere was a giant blue eye." - Death p. 350. I chose this quote because the ideology that no matter how much Hitler persecuted the Jewish population, they wouldn't go anywhere.