[Help] Styles are not applied

[NilsMoller]

I can't get the styles to work.
I downloaded the github-markdown-dark.css file and verified the HTML file can find it (which it can).
I copied the HTML in the README and use it as-is.
Something changes, because the position of the Unicorns and All the things texts changes, but no darkness to be found anywhere.

I'm not running any extensions, and this happens both in Firefox and Chrome.

What am I doing wrong?