So, no, it is not "easily configured in Firefox" or anything else running on the client side. When I visit various sites and have to squint at the text, that's 100% on the site designers. It may be fixable by various third-party extension hacks and kludges with numerous drawbacks of their own, but reskinning the site itself isn't something the browser can (or should) be expected to do.
Ideally, sites where the admins prefer light-on-dark text should follow Wikipedia's example, which really sets the standard IMHO, and give users a choice -- auto, dark, or light mode. Here again, 'mode' refers to an option provided by the site, with nothing whatsoever to do with client-side chrome. They are basically just giving you the option of using different curated style sheets, which is great.