Dark mode in documentation site

**Is your feature request related to a problem? Please describe.**

Currently we only added light mode because its better to not have dark mode than having a badly designed dark mode. But at some point we would need a good dark mode since that is expected from documentation sites

**Describe the solution you'd like**

I think first we'll have to play around colors and decide what type of dark mode we want. A full black dark mode something like vercel? or more like a dim mode?

Opposite to the the current only-light mode, https://v0.abelljs.org had 3 modes - dark, dim, light but then we end up with 3 versions where contrasts and aesthetics get difficult to maintain.

**Additional context**

This might need few iterations before it goes live on production so the development will likely happen in parallel branch for some time