How to activate new adminhtml theme in Magento?

Within config.xml under etc folder of your module, add the following

    <stores>
        <admin>
            <design>
                <package>
                    <name>NameSpace</name>
                </package>
                <theme>
                    <default>theme</default>
                </theme>
            </design>
        </admin>
    </stores> 

Assuming you created your theme under app/design/adminhtml/NameSpace/theme