The Dark Reader extension does that very well and even works on mobile (Firefox for Android). If you really want to do this without extensions, you could try a

The Dark Reader extension does that very well and even works on mobile (Firefox for Android). If you really want to do this without extensions, you could try applying a global CSS filter using one of the CSS files in the profile folder (I don't remember which, but I think it's userStyles or globalStyle.css - should be easy to look up). The filter to apply is first an invert(1), then a hueRotate(180deg) on top of it. It's not perfect, and it will have the opposite effect on pages that are already dark, but it's the best you can do without an extension.