dark mode background color

[billpebert]

Hi @adamwathan it's great to see the new version of the tailwindcss documentation page. but i have a simple suggestion, which i think it's better if use bg-gray-900 instead of bg-gray-950. the current background-color is too contrast with the text-color

bg-gray-950

bg-gray-900

and also it's nice if we keeping the dark/light mode switch 😉

thanks for your hard work on this tools 🍻