need to transfer ownership of site, need theme to carry over to new owners


I signed up for a yearly package in January. I am using the blogs mu child theme for a site I have built for work. I have made several changes to the CSS this theme. I am leaving the organisation soon, and they are very keen to purchase the theme for themselves. However, they don't want to lose the customisations I have made in the current theme. How can we arrange for them to own the theme for themselves, but keep my amendments?

This is a fairly urgent enquiry as I am leaving in less than a fortnight.