Finally I think I found how to.
1. My Account
2. Styles and Themes
3. Where the drop down option bar originally says "Browse all Themes", change to "My Themes"
Then you should see the option to edit your theme. Once you do that, you then change the theme in your group.
I hope that made sense :)
I have just created a new team and would like to create my own theme for it, can someone explain how to do this plzz?