Hello, I know this post is on the older side but I made a tutorial on how to make the background any solid color very easily.
Hopefully, this helps anyone who finds this thread in the future 
I’ll also include steps below for anyone who doesn’t want to watch a video.
1 - Delete default skybox
2 - Drag in exponential height fog
3 - Set Fog Height Falloff to zero
4 - Turn off atmosphere sunlight in light source
5 - Adjust fog instancing color to whatever you want