same theme with small differences per blog

Hi all;

I want to have all my blogs with the same custom theme I
created. I also
want, for example, the header picture to be different per
blog or to
style a specific blog element differently per user..

Note that my blogger admins do not change anything
themselves, I do all
the prep work in advance (I get the header image and style
requirements
from the blog admins).

What is the best way to achieve this?