This is a misunderstanding: “External” doesn’t mean coming from an external source like GitHub.
It simply means coming from the CSS file. Put a span into the div, write some text in the span and give it a text color in styles.css, since you can only use type selectors (One solution).
In contrast to inline styles and CSS defined in style tags that are on the HTML page.
So I stand corrected that only the CSS file is the solution. GitHub probably didn’t do it if you tried to link the raw file from the depository. You need to host the CSS somewhere.