All around this is a fun game, nice job :)
I noticed the background appeared to be a panel with an image on it. I suggest you checkout this article and look into using shaders for your background: https://www.codinblack.com/outline-effect-using-shader-graph-in-unity3d/
That article has helped me in the past, and I think it can help you too. Just a heads up, it might be a bit challenging because it gets into shaders.
Great job, and good luck!