Inject a CSS file in the DOM with the greasemonkey function GM_addStyle. Useful for loading a simple CSS file without embedding both css-loader and style-loader in your userscript.

npm install --save-dev userscript-css-loader

// ==UserScript==
// @name           Example userscript
// @grant          GM_addStyle
// ==/UserScript==

require('userscript-css!./style.css');