For all the people here using Opera GX, easiest way to get rid of this annoying thing is by disabling force dark pages. Go to the easy setup and look for a "Force dark pages" and press Configure below it. Then you can see the able and disable, put youtube in the disable list and then boom, it's gone. I was also very annoyed by this, but I figured it out by doing this. IF this doesn't work for you, well.. idk but this is the way that I found.