Websites with astoundingly poor font and color choices are very common. The reason content creators do that is a mystery to me. I suspect some website designers place greater emphasis on their own cleverness than creating a useful website. Maybe that's how they get paid. Perhaps they just have bad taste. Or they're color-blind. Who knows.
Apple Support Communities (this site) is one of the best examples, if not the very best, of a website that's useful and readable with a minimum of clutter and distractions from the actual content people are seeking.
For websites lacking in that area, Safari offers Reader Mode.
Reader Mode also allows some customization for font and size selection and background color if black on white is too harsh for your taste.
Of course Accessibility Settings is an option but it applies to everything system-wide, not just Safari.