There is a powerful feature 'Skin Inheritance' thats got introduced in 7.2 .
For our skin , we can add parent skin(s) so that all the mixins and formats from the parent skin got inherited by our skin. And we can override or delete the mixins/formats. Even stylesheets also got inherited by our skin.
In skin rule, there is a 'Inheritance' tab ,so that we can add parent skins .