In The Book Thief, what is Markus Zusak's message about the theme of outsiders vs. insiders?

The message is developing awareness of how noxious stereotypes intensify xenophobia and racism, shattering the dignity of the victimized communities.

And shows the difficulties illustrating a particular culture, when you're not a member of it.

Thereby how important it is to grow "cultural consciousness" and learning about cultures different from our own; since it consolidates the power of humanity, to come together in times of crisis.