Add dark mode support for homepage

[gprasanth]

Tried adding dark mode support for the docs on the underscorejs.org homepage.

Here's how it looks before and after:

The code is available in a very short gist here:

doccy-dark.css

To submit a pull request I am not sure where to add this... in index.html in source root? or in docco.css?

In any case, this can be tweaked as required and added. Also, the logo lost the signature blue color, but can be avoided if an alternative logo image with white text is available.

[jgonggrijp]

Nice suggestion, thanks!

[itsdeekay]

@gprasanth You can raise PR . Recommended would be to use in index.html itself as all of the existing code is inside it. Also it ca be separated in docs folder and with specified folder.