1. This is not a feature-request forum. There's a dedicated support page for things like that.
That said, you can easily do what you want in Settings/Custom CSS, enter this line:
body {background: #222 url("https://picsum.photos/1800/1200/?random") 0 0/cover no-repeat;}

2. Please understand, that I'm doing that for free, in my spare time, and you're not paying a cent for it. So I'd appreciate if you didn't put me down because in the free software that I gave you for free, I didn't include all the functionality that, I didn't know, that you wanted.
If you're looking for a feature that is not there, please add a feature request in project's github and I'll do my best to add it for you.