Sorting out exactly what a WordPress theme is ain’t so easy. The very easy mistake to make is to regard the theme as a template in HTML webpages. But a theme is much more than a colour scheme and a pretty picture.
The Theme is a Machine
Your wordpress theme may have a special box in the admin area for your Google Adsense Publisher code, it may have special header customisation options which even edit the header graphic, it will have a way of choosing whether and where you have sidebars and photograph galleries.
There’s a lot of work going on behind the scenes and even many theme designers seem oblivious to the real needs of the blogger and wish to “express themselves” artistically.