Manual light/dark style override

[cassidyjames]

From elementary-Inc/blog#12

Should be relatively easy to do with lightweight js and local storage

[michallepicki]

I could pick this one up someday but if someone else wants to do this, here's how it's done in Elixir's ex_doc documentation generator. Effect here - there's a "Toggle night mode" link in the footer. You would just need to use plain js instead of jquery.

[moshenskyDV]

I've created PR #17
Maybe I should wrap the night mode code to scss mixins to prevent code duplicate? Or right now is ok?

[cassidyjames]

@moshenskyDV thanks! I've left a review on the PR.