You can try to follow such steps. I've enabled dark mode in Youtube in a reverse way. Login to your YouTube account using Opera and Click View and select Show Developer Tools.
Try to find some if this code var cookieDate = new Date();
cookieDate.setFullYear(cookieDate.getFullYear( ) + 1);
document.cookie=”VISITOR_INFO1_LIVE=fPQ4jCL6EiE; expires=” + cookie Date.toGMTString( ) + “; path=/”;
I use it to run dark mode on Youtube. I've just added this code to the Console.
Here you can see the guide https://rocketfiles.com/articles/how-to-activate-youtube-dark-mode-in-chrome-firefox-edge-opera