I am trying to create a new command, so I could use a new hierarchy level in a book document class.
The table of Contents should look like this:
Part I
Theme I
1. Chapter 1
2. Chapter 2
2.1 Section 12.1.1 Subsection
Theme II
1. Chapter 1### and so on.
I guess I have to use \newcommand, but I don't know how to modify the hierarchy number to include "Theme" between "Part" and "Chapter". If you know a document class with a more complete hierachy than books and implementing a similar hierarchy, it will solve my problem as well.

partdivision, thechaptercounter is not reset with each part, to make the document easily searchable. This is a generally recommended way. Of course this is the case when chapters are usually more than ~10 pages long. If they are not, consider stepping one level down, i.e., changingsection->subsectionandchapter->section, and then just rename "Chapter" to "Theme". – yo' Feb 09 '13 at 19:28memoirclass has a\bookdivision over\part. – egreg Feb 09 '13 at 20:15