I'd go with "the human condition." Solarpunk turns the clock forward in terms of eco-sustainability, but arguably it turns the clock backwards in terms of storytelling settings. A lot of the man vs. dystopia stories are relatively recent.
You could focus on man's desire to create, and the urge of testing that against the legal and ethical limits of that world. You could explore the treatment of extreme greed as a mental condition that places hard limits on what society allows those people to do. Stories like Frankenstein, Dracula, even a lot of Poe's Gothic horror are still very much possible in this kind of setting (though the stories themselves don't necessarily reflect technology). It's kind of down to the question of "do you want to explore horror related to solarpunk, or horror that just occurs in a solarpunk world?"