What is the theme in Deep Wizardry by Diane Duane?

At it's very core, this is a tale of good against evil at the most base level. Kit and Nita are wizards, powerful because of their youth, and effective because of their fearlessness. Their predisposition to wizardry allows them to find the manual that quickly becomes their bible. In the beginning, their activities are kept secret, but when Nita's mother begins to question the inordinate amount of time that she is spending with Kit, believing that perhaps they are becoming physically involved. When Nita and Kit become entangled in an undersea assignment that keeps them from making their curfew on more than one occasion, Nita's parents decide the best course of action is to ground them both until further notice.