Hey, Shmooper. Is your day going too well? Is everything a bit too happy? Well, take a look at this paragraph and try not to feel a welling sense of nauseated sadness:
"Gonna have a sweet sunburn tomorrow." He knew what he was doing. Trying to convince himself that things weren't so bad—that he wasn't losing it up top, that his anger and headaches weren't going to hinder his concentration and focus and everything was going to be fine. But the effort seemed pointless. (51.3)
Yep, this pretty much sums up the tone of this book. It's pretty dismal, especially because we as the readers know our main characters aren't going to fare well. For Mark and Alec and Trina, the end is near. They know it. They know they'll get the Flare sooner or later—even when they joke they can feel a sense of sadness (and a sense of the Flare) creeping around in the back of their skulls.
Sure, Mark and Alec have some great moments together. They laugh, they bond, and they joke with each other. But all of this is often cut short by realizations that they're going to die.
Sound like the apocalypse is nearing? Yes. Yes it does.