Other than removing the extraneous whitespace from the example's HTML and the body rule from its CSS, it seems to work fine to me.
You're probably trying to use the LESS code, rather than the compiled CSS. By default, a CodePen example (a.k.a. a Pen) show the raw code given by the developer, which isn't always HTML, CSS, or JavaScript. In the header for each section it shows if the code is actually HTML, CSS, or JavaScript or if it's in some other language by adding the name of the other language in parenthesis after the title. In this case, the author used LESS for the CSS code section, which is a pre-processor language for CSS. Click the "View Compiled" button in the CSS section's header to see the CSS version of the code.
Anyway, you should have something like this after viewing the compiled CSS (and removing the body rule).
HTML:
<div class="wrap"><button>What's this do?<span class="left">It doesn't do anything, don't bother clicking on it.<span class="nub"></span></span></button></div>
CSS: (the @import rules must be the very first thing in your Story Stylesheet, the rest can go wherever)
Comments
You're probably trying to use the LESS code, rather than the compiled CSS. By default, a CodePen example (a.k.a. a Pen) show the raw code given by the developer, which isn't always HTML, CSS, or JavaScript. In the header for each section it shows if the code is actually HTML, CSS, or JavaScript or if it's in some other language by adding the name of the other language in parenthesis after the title. In this case, the author used LESS for the CSS code section, which is a pre-processor language for CSS. Click the "View Compiled" button in the CSS section's header to see the CSS version of the code.
Anyway, you should have something like this after viewing the compiled CSS (and removing the body rule).
HTML: CSS: (the @import rules must be the very first thing in your Story Stylesheet, the rest can go wherever)