Log in to Admin:
Navigate to Themes, then click on New in the upper right.
From our list of available stock themes mouse over the theme you want to add. Once the theme highlights as shown in the screen shot below, you can preview it, give it a new name in the "New Theme Information" section. Once you've decided on a them and given it a name, click the ADD button to select it.
You will now be taken to the start page of your new theme.
Once your new theme is ready you can activate it.