This sounds like a conflict between Opera GX’s Force Dark Pages feature and YouTube’s own dark theme or a third-party extension, so the best fix is to disable Force Dark Pages for YouTube specifically, remove any dark-mode extensions, then reload YouTube and re-enable its native dark mode from settings.
Looks like this is a known duplicate issue with Opera GX where background processes show separately in Task Manager, so it’s usually normal behavior rather than a bug.
At the URL opera://settings/profiles, if you click the 3 dots to the right of the profile and choose "create shortcuts", do you get a desktop shortcut that works fine at least?