How do I add an external css style element to a div element in a custom widget?

I created a custom widget using the App Store Widget Boilerplate and now I have this template:

Grid.html

<div class="widgetname">
    <div id="grid" data-dojo-attach-point="gridNode"></div>
</div>

And I want this grid to use the css style element ./ilb/dgrid/css/grid.css#.dgrid.

I'm not familiar with css, certainly not the jargon, so the only thing I have come up with so far is 'loading the css'?

I got the idea from the basic hello world page and I don't know what to do next.

Grid.js

define([
    ...
    "dojo/text!GridGenerator/widget/template/GridGenerator.html",
    "dojo/text!GridGenerator/lib/dgrid/css/dgrid.css"
], function (..., widgetTemplate, dgridCss) {