Just for anyone else that needs this, the javascript snippet you need to insert in double quotes is as follows:
"var nameFileCSS = 'yourfile.css';
var realUrlCSS = this.runtime.getProjectFileUrl(nameFileCSS );
var styleCSS = document.createElement('link');
styleCSS.rel = 'stylesheet';
styleCSS.href= realUrlCSS ;
document.head.appendChild(styleCSS );"