If you search "how to turn on dark mode" in chrome, it'll change to dark mode. Or you can change it in settings. For the numerous websites that don't care, extensions exist to force them. Midnight lizard is my favorite.

Edge has it in edge://flags no extension needed. Firefox & brave requires settings and extension like chrome. I haven't used opera or Vivaldi in ages to help with them, but they will have an option because customizability is a key part of their selling points