I'll agree, this is one of the things I don't like about macOS. It looks like in the past there were hacks to do this, but Apple broke them. (Similarly, I'm so glad there's a hint of colour in the sidebar in the Finder again; I hate when Apple says, "We've got two indistinguishable shades of grey; one for available, and one for inactive. You can take it, or you can take it.") As to the point about setting a desktop b…
At some point, I realised that if I just take a deep breath, slow down, and accept that I can modify myself to align with my tool rather than the other way around, a lot of friction is removed.
I set my background on every device to a solid black, and it's now three years since I changed them. My trello boards are still blue background. My calendars are whatever the default theme is. It's sometimes nice to tweak the tool for your need, but it's also sometimes nice to understand that everything doesn't need to be a huge annoyance. One can just adapt and move on.