It´s possible, but it´s not very easy to be done. The first thing you need to do is understand how a grocery crud theme is implemented. The themes are in .../assets/grocery_crud/themes
You will have to look the code of one of the three oficial themes available today:
https://github.com/scoumbourdis/grocery-crud/tree/master/assets/grocery_crud/themes
You could pick one of the themes and implement your changes on top of it. I recommend you try first with the twitter-bootstrap theme (i guess it will be easier to implement RWD on top of it).
If you obtain sucess doing this, i would love to see the results. If you do a great job, you could even add your theme to the grocery crud theme package. It would be a great addition to grocery =)