Here's an add-on that does exactly that: https://github.com/mdn/webextensions-examples/tree/master/private-browsing-theme https://github.com/mdn/webextensions-e

Here's an add-on that does exactly that: https://github.com/mdn/webextensions-examples/tree/master/private-browsing-theme https://github.com/mdn/webextensions-examples/tree/master/pr... 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.