Yeah, I remember working on a project like this: those illusions are not trivial. Nice that you managed to pull it off without any detrimental bugs.
If you want some pointers on visuals, first off, there's nothing wrong with monochrome. What you have here is a nice starting point, but it feels a little flat, and not because you're using an orthographic camera. What I mean is that from the way the game appears there is very little meaning to be made of it. That's part of what visual identity is: extracting meaning from visuals.
To establish the vibe you already have, one of abstract calm, you could add some dark grey background objects and elements. These could be unity primitives like lines, cubes, or spheres, or they could be more complex shapes or other cool impossibilities like the Necker cube. These objects could slowly rotate/float against the black backdrop, just to give the space some motion.
Since you're using Unity, there are a few tools that could actually help you. Particles are useful for giving a sense of atmosphere, literally; particles help encode what the air is like in the space. Slow wandering point particles could resemble dusty air, floating wispy clouds show humidity, faster curvy lines show wind. Particles can also be used to show the weather, like rain or snow. Bloom is a also neat tool I would use here; it's a post-processing effect that can emphasize light by layering a little blur over the screen to give a glow effect to anything bright. If you're struggling with aesthetics, definitely learn how to use Unity's particle system and post-processing (they make anything look 1000 times better as long as you don't overdo it).
If you need some inspiration, look up some abstract art from Frank Lloyd Wright. I get that the designs might be a bit too complex for this, but I think they're a good starting point for some neat shapes that compliment what you currently have. Also, if you're interesting in making abstract game art yourself, learn to use Inkscape. I think it's really good for making abstract, geometric art.
Hope this helped! Best of luck!