In this book, "Night" represents darkness as in wickedness or evil.
One of the main themes of Night is Eliezer's loss of religious faith
“I did not deny God's existence, but I doubted his absolute justice.”
― Elie Wiesel, Night
Blessed be God's name? Why, but why would I bless Him? Every fiber in me rebelled. Because He caused thousands of children to burn in His mass graves? Because he kept six crematoria working day and night, including Sabbath and the Holy Days? Because in His great might, He had created Auschwitz, Birkenau, Buna, and so many other factories of death? How could I say to Him: Blessed be Thou, Almighty, Master of the Universe, who chose us among all nations to be tortured day and night, to watch as our fathers, our mothers, our brothers, end up in the furnaces? Praised be Thy Holy Name, for having chosen us to be slaughtered on Thine altar?
Another theme would be the inhumanity of towards other humans.
I told him that I did not believe that they could burn people in our age, that humanity would never tolerate it . . .
Humanity? Humanity is not concerned with us. Today anything is allowed. Anything is possible.
We must take sides. Neutrality helps the oppressor, never the victim. Silence encourages the tormentor, never the tormented. Sometimes we must interfere. When human lives are endangered, when human dignity is in jeopardy, national borders and sensitivities become irrelevant. Wherever men and women are persecuted because of their race, religion, or political views, that place must—at that moment—become the center of the universe.
They are committing the greatest indignity human beings can inflict on one another: telling people who have suffered excruciating pain and loss that their pain and loss were illusions. (v)
LAstly, the importance of family bonds
I didn't know that this was the moment in time and the place where I was leaving my mother and Tzipora.
My hands were aching, I was clenching them so hard. To strangle the doctor and the others! To set the whole world on fire! My father’s murderers! But even the cry stuck in my throat.
My father was crying. It was the first time I saw him cry. I had never thought it possible.
Elie'S purpose of writing night was to Create a memoir so others can read abd prevent anything similar to the holocaust happen again. Also, to show to take care of what you have now because at any random moment something can occur.