I didn't have that option in the latest Play Books, but it did have a Sepia, normal, and another color theme. Google could have hidden it like they do everything lately. Right now I'm happy to have my 3-D library view back over the pastel flat junk. I wasn't aware the night mode even existed until I downgraded.
"If you treat your system like a kiln, don't be surprised when a brick comes out the other side."