First, welcome to the community portal. About your question, look in the design folder(s) for the admin2 design. You will find the admin templates in there. To properly use another admin design is to copy the admin2 design, and rename it. Then change the used admin design in your ini files and change any template files in your new design folder.
There are also some good articles about designs. Let me/us know if you require more information.
Kind regards, Robin
Board member, eZ Publish Community Project Board - Member of the share.ez.no team - Key values: Openness and Innovation.