After some digging I did find the way to do it. I'm not sure if this is the intended way, however it works fine.
Put this code in your app.js and the simulator will instantly refresh with changes when saving the CSS file.
var PX = require('com.pixate.framework');
PX.styleSheetFromFilePathWithOrigin({
monitor: true,
origin: 0,
filename: '/absolute/path/to/app.css'
});
// you can add multiple css files to monitor
PX.styleSheetFromFilePathWithOrigin({
monitor: true,
origin: 0,
filename: '/absolute/path/to/another_file.css'
});