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.
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.