I use my OS in dark mode but I don’t enjoy most websites in dark mode as it’s very hard for me to read them. Some sites give options and some do not. Also some sites such as DuckDuckGo are now no longer usable in Incognito mode or with the Brave option to clear cookies since I must now save cookies to use DuckDuckGo in white mode. Perhaps you could add a brave://flag to disable this reporting? Yay to dark mode but Nay to read my mail and sites in black.
I think this would be a good idea, as anything that a website can query is another way to create a traceable profile of a service’s user base.