**Arclight is about light and dark**
The novel's world is divided along a permanent meridian line. The humans live inside the Arclight, protected by lamps that create a canopy or perpetual daylight, while their enemy - the Fade - live in a state of perpetual darkness. Everything seems to be fairly clear cut and defined, but it's not quite that simple. In fact, rather than light and dark, it might be more accurate to say that the novel's world is separated into yin and yang, because neither the light, nor dark, is absolute.
Marina - the main character - knows that she somehow escaped the Fade's domain, so there has to be more beyond the Arclight that what can be seen. Likewise, her sanctuary comes with shadows and hidden corners, which harbor secrets she can't imagine.
There's also an intangible darkness that works its way into the characters' lives, no matter how bright the lights burn. Marina runs from it, afraid that her time in the Dark has somehow tainted her; if she can't stop it, it will completely consume her. Tobin - the male main character - walks a similar path, but one much closer to the darkness, overwhelmed by the loss of his father. His light's going out, and he's not even sure he wants to save himself, anymore. It's easier to give into the rage and hate, and vent his frustration when he needs to.