Hi! Thanks for playing with our game!
I made the levels in Blender and created a base for the texture and edited it in Krita. The base usually consisted of a few colors just to mark certain areas of the level. For example, in the gallery level we wanted a noisy texture for the floor but a flat color for the wall so the pictures are more visible.
I found the halftone effect in Krita, and I started to use it a lot to create more and more distorted patterns and mix it with the hand-drawn elements. Sometimes it was easier to do something in Blender, so I kind of worked back and forth in the two software. In the end all the textures are black and white.