It would be very handy if you could automatically see what happens when you add some custom css or add a class. Now you have to type something run locally have a look, change it again, run again, have a look again and so on. I would suggest you could do this in the webmodeler but then you need to be able to add custom css or a class to the webmodeler.
Advantages:
Not exactly 'integrated in the modeler', but have a look at Styling with Gulp.