I never thought about theme, not for years. I always went for plot and characters. Now I think about it more deeply because I recognise that having some idea of theme feeds into the plot, adds layers and resonances that wouldn't have existed had I not been aware of it. It adds richness.
I write supernatural fiction and fantasy but my themes are usually based around family and relationships.
This current WIP is really about families - how your upbringing can leave you damaged, the role of biological family compared to adopted family. How you can create your own family by finding people who care for you, perhaps more than your blood relatives.
My last book was at least in part centred around loneliness, mental health, isolation, the importance of friendship.
At the same time, both books were about ghosts and demons and psychotic Victorians!