Theme
There isnt really a theme that stands out but one that I can think of is that love can pass all tests. One piece of proof is that Annabeth flies hundereds of miles to reunite with Percy Jackson. Second piece of proof is that Piper fights Achelous to save Jason who she loves. Percy Jackson tries his very best even when he knows odds are against him to try to save Nico DiAngelo. That is the only theme that i can really think of that could make sense.