Yeah I just tried that and it changes the private window to light as well. I guess it doesn't matter much cause i don't use private much, but it's weird they have removed that distinction now.
If you want that kind of distinction, it should be pretty trivial (less than a dozen lines of code) to implement a dynamic theme to that effect. We've got an article in the https://hacks.mozilla.org queue about exactly that -- stay tuned :)
It's not currently packaged and uploaded to addons.mozilla.org, but you can try it by cloning the repo, navigating to `about:debugging,` and clicking "Load Temporary Add-on." Select the `manifest.json` file and you're good to go.
