Hi Max,
You can not have a complete black website without adding CSS code when using the Ethos stack. We suggest that you use the Integrity Stack. Then you will have an option to set the website into dark mode by going to X/Pro > Theme Options > Integrity > Design:
After that please kindly go to X/Pro > Theme Options > Layout and Design and scroll down to see the background related options which you can set.
kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case if you still have problems.
Thank you.