What kind of artistic style categorises such paintings?

The artist, Debbie Criswell, calls it Primitive Folk Art.

Building on Richardson's answer, a search of 'gothic landscape folk art' yields similar results for me!

And I think this effect could be achieved with both oil and acrylic paint on either canvas, paper or if you like, wood.

As for the techniques for this painting, it seems more like oil than watercolor. If you wish to paint this way it appears they use a large square brush and a smaller pointed one. There is not much mixing of colors or blending. They appear to take a single color and place it solidly on the canvas. Except for in the moon and foliage, where they use a large brush in the moon of varying gray shades, and a small one in the trees of darker colors and lighter. And then of course some light shading on the street.

As for the style: it appears to be a contemporary landscape modern. Or, if you want, a take on Folk art, or contemporary fantastic realism (though that'd be a stretch)