Why not create a sub-theme? http://drupal.org/node/225125

Ted

On 7/18/2011 9:04 AM, Ms. Nancy Wichmann wrote:
If I change the Seven theme files, when Drupal 7.5, and all releases above it, come, I will have to do them all over again - if I, or who ever takes over after me, remember. A module with nothing but a hook_init() would work, but seems overkill. Maybe the best thing is to just not use an admin theme.
 
Nancy
 
Injustice anywhere is a threat to justice everywhere. -- Dr. Martin L. King, Jr.

From: Earnie Boyd

> Is there some easy way to give Seven (the admin theme) another CSS file (or setting to it) that it won't overlay on upgrades?

Have you considered adding the CSS changes to a module?  How about
changes to the CSS for the theme itself?