react-magma-docs: Implement Dark theme

[silvalaura]

Let's investigate (and implement if possible) how much effort it would be to have the ability to toggle light and dark mode in our documentation site.

The benefit of this is to showcase how this can be done for other applications that may want to implement a dark theme.

Questions:

• Should it just use isInverse or be a brand new theme that uses the isInverse colors?