Extract hard coded values into variables

[iamkoch]

I like that this is opinionated, however hooks to change things like max-width would be useful.

I've easily overridden the background colour by setting --bg, so it'd be great to extract other, more overarching, variables out into a place where they can be set once.

For example, a wider header with less bottom padding, a wider body, etc.

[louismerlin]

That's a great idea, I'll try my hand at this issue at some point. Feel free to submit a PR though if you have the time 😄