Dark mode

I searched the code base for text listed on the front page like:

"Easy file sharing from the command line"

and

"Preview your files in the browser!"

but no hits... which is a bit surprising.

I want to modify the CSS coloring a bit to also support dark mode:

These are some of my current overrides
```css
.body
{
background-color:black;
color:silver;
}
#features{
background-color: #222
}
#reviews{
background-color: #222
}
```

Where/how is this located in the repo so I can create a PR for it?