Magento do not provide any easy way to change its admin theme or template. Some time back I had a task to change size of an image displaying for category in admin panel of magento. I could not just overwrite or change the default core admin theme. After some searches I got a fine solution. That goes with Magento quite well.
Here is a step by step tutorial to be able to change the theme of admin panel or say backend of Magento.
Create a new module. Yes, you have to create a new module for that.
So go into your local folder of code and create the following folders. App/code/local/Packagename/AdminHtml/etc
Create config.xml in etc folder. In this file write the following code.