Although it's late, I'd like to provide this for anyone looking for a solution for this problem, Opera actually has a feature where it can force dark mode on any website. To fix this, go to the website right click, and you will see Disable Force Dark mode. If you press that, then the website will go to its original state. Keep in mind that If you are already in dark mode on the website, it will do nothing. You would have to search up how to turn off dark mode on that website like, say, for YouTube, but if that website is like Google Docs and doesn't have a dark mode then this should work. You're welcome if this helped!

Although it's late, I'd like to provide this for anyone looking for a solution for this problem, Opera actually has a feature where it can force dark mode on any website. To fix this, go to the website right click, and you will see Disable Force Dark mode. If you press that, then the website will go to its original state. Keep in mind that If you are already in dark mode on the website, it will do nothing. You would have to search up how to turn off dark mode on that website like, say, for YouTube, but if that website is like Google Docs and doesn't have a dark mode then this should work. You're welcome if this helped!