I am getting a lot of emails and comments regarding dark and light modes. Users seem to think that a theme designer has control over the New Page/Home Page color. Can someone give me a good answer? I, too like a dark New/Home Page. Perhaps theme designers could get the choice of that when they submit a theme or is there a Firefox setting (I can’t find one) that we can either use a dark or light mode? I hate when I can’t answer a users questions to their satisfaction so I am hoping someone can help me out on this. Please…
The colors of the new tab page can indeed be controlled by a theme: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme (look for ntp_background and ntp_text)
However I think only Firefox Color currently exposes this in a theme editor UI, and not the theme editor on addons.mozilla.org.
Thank you. That isn’t going to help the usual theme user, but it gives me an explanation for it. Perhaps Mozilla will add the Home/New Page color addition to the item that theme designers can set? Probably just wishful thinking, but you can always wish!
I noticed with Mozilla Firefox’s recent update, my favorite theme suddenly is in dark mode with pop up windows and certain webpages I go to, like youtube… I don’t like dark mode…
Yeah this is really beginning to annoy me, a lot of themes that don’t look very dark at all are being considered dark simply because they have white text, and this is sending a signal to a lot of websites such as YouTube or Twitter to turn on their dark mode, which I don’t like as I am not a fan of dark mode for most sites. I guess I can fix it for the themes I’ve made, but for themes I haven’t made, I can’t. Really frustrating. 
It seems like a pain to manually set all sites to not use dark mode, so I guess I will just have to use themes that are recognized as light for now. PLEASE make a feature to manually set the browser’s ability to send a signal to dictate if the theme of a site should be light or dark separate from the browser’s theme.