The applied translation "پوسته" is appropriate for "skin" which is another popular term for app appearance. However considering the etymology of the word "theme" which relates to the "root" and "base" of a matter it is better to use some corresponding word in case the word "skin" would be used in the future so "پوسته" could be a translation for it. Thus the word "بن" or "بن‌مایه" is suggested and to deal with the length of the word I selected "بن" that is even shorter, although "بن‌مایه" is no less appropriate if not better.