Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Wondrous
Throughout the book Oppel, writing through the eyes of Matt, maintains a sense of incredulity at the beauty of the world around him. His descriptions of the ship—which to us laypeople might seem excessive or even tedious—are done in such a way as to evoke the grandeur and wonder that it all somehow works:
Down the ladder I went, through the webwork of alumiron beams and bracing wires that gave the Aurora her rigid shape. On either side of me hung the walls of one of the enormous gas cells that kept us aloft. Their fabric, a miraculous substance called goldbeater's skin, glistened and rustled ever so slightly as I passed, like something alive and breathing. (1.42)
See? It's almost reverential—like if there were a church that worshiped bags of gas Matt'd be in the choir singing at the top of his lungs.
Oppel maintains this awed voice throughout the story. Instead of describing the cloud cat as mangy, smelly, or weird looking, it is marveled at for its beauty. And even the pirates, who are supposed to be the bad guys, are described with a sense of wonder.
This allows us as the readers to have the same experience that Matt does as he takes in all of the things around him. We are tourists, stumbling through an inspiring new world with round eyes and mouths hanging open at the beauty of it all.