@Pyll0 Thank you for the reply. However, running the chromium flag has miraculously turned all websites dark (i.e. white text on black) and with Opera's own dark mode that is all I need.

Many thanks, leocg. I had no idea a chrome flag would work on other browsers. But it does. That is brilliant!

Sorry, I didn't make myself clear. I know Dark Reader will do that in Chrome; I'm looking for an extension to do the same thing in Opera.

Is there any add on or extension that will put any website into dark mode? So that text will appear in white on a black background, the way that Dark Reader works on Firefox and Chrome.