Does each season of Dexter have an overarching theme?

Yes, each season has a theme. From a post by EnormousHatred on reddit:

Season 1: Individualism. In the beginning Dexter followed Harry's code to the letter and never really let his own feelings get in the way. He never really questioned a way of life that came from within. Brian represented the antithesis of Harry's code; he was a free spirit with no rules. By the end of the season, Dexter's strict adherence to Harry's teachings ended up costing him his brother, "the only one he ever wanted to let go".

Season 2: Possession. Lila was the embodiment of dependence, she coveted Dexter to the point that it nearly brought down everyone around her. The opposite action is to let go, to free oneself of the bindings, the vices that sustain yet also chain us. Dexter struggled with this when he had to make his toughest decision yet regarding Doakes: "Kill me now or set me free!" It was either give in to his greatest weakness, the need to kill, or set him free and ultimately risk losing everything. Doakes brought up suicide, and even planted seeds of regret and shame in Dexter's mind, causing him to consider turning himself in—other ways of letting go. (Addiction lies in this same sphere so you were pretty much on the money here.)

Season 3: Trust. This mostly goes hand-in-hand with friendship, but it extends beyond that somewhat. There were parallels drawn between Dexter's relationships with Miguel and Rita. Friendship and marriage are ultimately built on the same foundation of trust.

Season 4: Responsibility. With family comes a great need for responsibility and priority (see also care, neglect, nurturing). Dexter eventually learned the hard way that you can't just make perfunctory appearances and not expect things that have been built up to degrade. (On a side note I think this was eventually illustrated in a clumsy way with the twist ending, but at least the point was clear.)

Season 5: Altruism. Story-wise it was sort of an extension of the last season because Dexter was dealing with guilt over his brief and neglectful marriage. The things he did for Lumen were essentially an act of contrition. He couldn't make things right with Rita, so he took in a battered and broken individual and selflessly took on her own darkness. He might have hunted these same individuals had he not met Lumen, but in the end he took his hands off the steering wheel and let her solve her own brief taking-on of the "Dark Passenger".

Season 6: Faith. Dexter never really takes on a spiritual side, but he gains a respect for the concept of faith. He didn't start to question whether there was a "God" per se, but Brother Sam was able to help him discover a capacity to trust that things will work out when they lie outside your control. Control is important to Dexter, so when Harrison was in the hospital, he felt helpless because there was literally nothing he could do but wait. Travis represented the dark side of faith by taking his Lord's name in vain. He was a person who chose to interpret mere words in a radical, concrete fashion, absolving himself of responsibility by saying this is "how it's supposed to be", while missing the bigger picture unlike Sam. Ironically, Dexter found himself hoisted by own petard when he decided to faithfully disregard the minute possibility that Debra would visit him at the church.

Season 7: Love. Pretty obvious so far, though I don't mean that in a bad way. The dangers of love make good and bad people alike suspend rationality and integrity; Debra's love for Dexter is great enough that it drives her to cover for his heinous crimes; Dexter can't bring himself to take out Hannah despite fitting his code quite well; Isaak chooses to pursue a frivolous vendetta, ignoring the fact that Viktor, objectively, committed some terrible acts. More on this one as the season churns on...

Another user, PunchingClouzot, replied:

Pretty interesting. I'll bet Season 8 will be "Sacrifice" to complete the pattern.

I would argue that, if EnormousHatred's theory is correct - and I wouldn't doubt it - there's a an arc on the discovery for human empathy. Individualism being his status quo, possession a natural step to follow which ended with a need to settle hence "trust" (the first positive step). Everything else leads towards a more empathic character BUT his nature cannot be denied hence why a conflict between his new self and his old dark passenger can lead to one ultimate altruistic act - sacrifice.

Maybe Dexter was completely human this whole time and Harry was fooled into turning Dexter into a guinea pig for Vogel. Dexter was brainwashed from a toddler until his first kill at 19. His father is shown throughout the show commenting and questioning Dexter's motives and methods, this could be how the viewer is made aware of this. Maybe Dexter could love normally but was taught to believe otherwise. The more time he spent in his "cover life", Dexter realized that it was actually what he wanted and could make for himself. The last season sums this pretty well. The scene where he heads right into the hurricane on the Slice of Life could symbolize a cleansing, or "baptism" and he is born again into a new life. (Previous seasons of the show depict Dexter trying to grasp religion and faith.) Dexter's boat was destroyed, as was his former self that had been created and his life was reset to a location and scenario painted with past events that would have stayed with him. #1 chainsaws (what he was taught "made" him who he was) #2 mountains (last conversation with Deb she discussed climbing them)... which leads into the last scene of Dexter working as a lumberjack in the mountains. Maybe he struggled to shore before landing in Oregon and had amnesia, not letting him remember his old life but a few clues as to who he was/where he belonged...