Need to make theme Responsive (correctly on mobile devices)

Hi,
I’m looking for someone to handle this for me.
There is a live site that has a custom edited version of the twentyeleven theme that won’t display correctly on mobile devices.

We inherited the site and theme so aren’t very knowledgeable in updating it to be responsive through css etc;

We are a web design company so please respond if you think you are experienced enough to handle this job. There would need to be a quick turn around and I think it would be an easy enough task for someone knowledgeable in the area.

Regards,