Step 1: Log in to DNN7 site as a host user.
Hover on Admin link over control panel and click Pagemanagement.
Admin >Page management
Step 3: Modal popup appears click on Advanced settings tab.
Step 4: Advanced Settings > Appearace
Step 5: Scroll down to Page skin in same Modal popup under Advanced Setting tab.
Step 7 : DNN will ask you for confirmation to add skin you selected to descendants of the parent . Click on Yes Button
Step 8: Success message will appear saying Design copied successfully.
Step 9: At-last update the page by pressing Update page button at the bottom of the modal popup window
That's it guys. This is the procedure to set a Different Skin for Portal Admin Only in DNN 7.