So far, it seems to work as intended. There's a disclaimer that it might be unstable for Opera, but I decided to take the risk because I absolutely cannot stand Google Docs without dark mode. It's a crime that Google doesn't natively support a dark mode for their desktop sites.

I like the ability to customize it so much, but the only option that really works for me is the Invert Colors one; the custom background colors don't change the color of the actual "page" I'm writing on, just the background. And with Greyscale enabled, it's very difficult to see any highlighted sections or spellcheck underlines, though I'm not a huge fan of inverted colors. It's a small price to pay to not be blinded while writing, though/

Thank you for this.