There are some horror-adjacent themes, like the occult elements intrinsic to the invocation ritual, as well as some influence on my own writing from writers known for horror-- specifically Lovecraft and Ryukishi07. Beyond that, it's mainly an aesthetic label.
I think it's really easy to get carried away in horror and create something that ends up being just kind of gross. That's something I'm consciously trying to avoid here.
Subjectively, I would not say Reality Layer Zero is particularly scary, although there are certainly tense moments. It's my hope, as well as my intention, that the evocations of horror are more in the vein of "gothic horror"-- magnificent and gloomy, perhaps, but not scary in a way that might meaningfully put-off a contemporary audience sensitive to such things. Your mileage may vary, of course.