Solid black is a bit harsh on the eyes. I use a solid mid-blue on both phone and computer screens.

I grew up when computers had to work hard to maintain cute background images on computer screens. If the background image was covered most of the time anyway, why waste scarce hardware resources to maintain it? So I and many others made it a policy to use solid color backgrounds only, never an image.

I did this years ago. It's the only thing that makes sense on a battery powered device

My timeout/login screen is still a full color photo of my loved ones but it is only displayed momentarily on occasion.