Implement dark theme

[sebaplawner]

I've made a simple stylesheet using CSS for Mumuki where I apply a Dark Theme to the default page in order to make it more 'captivating'.

By the moment, it's uploaded to an extension used by most browsers called Stylish.
Easily downloaded for Chrome, Firefox and Opera.
https://userstyles.org/

Once installed you can install the style from here:
https://userstyles.org/styles/125847/mumuki-dark

I'm leaving the source here, nevertheless, you can download it from Stylish as well:
mumuki-dark.txt

[flbulgarelli]

Hi @sebaplawner.

Although we are not going to change the main theme, we think you did an awesome work here. We are probably going to release a community site eventually, so we will definitively link your stylesheets.

We will close this issue when that happens.

Thank you! 😄

[flbulgarelli]

Related to #688