I use Opera GX everyday, and sometimes when you suddenly get a bright light in your eyes cause you open a new tab on a super flashy white page (i mean search engines, for example), maybe you would like to quickly get dark page. So why not to place a button in the sidebar to switch between light and dark mode easily.
It can works like the background music button, just switch it on or off by a simple click on it.
Oh by the way, Opera GX is a really good web browser with a good potential, for me
Have a nice day on the web! o7