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Magento allows customize admin theme. It’s like new theme for frontend but needs specific configuration in config.xml.
Below is an example:

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <modules>
        <YOURMODULE_Adminhtml>
            <version>0.0.1</version>
        </YOURMODULE_Adminhtml>
    </modules>
    <stores>
       <admin>
           <design>
               <theme>
                   <default>YOURADMINTHEME</default>
               </theme>
           </design>
       </admin>
    </stores>
</config>