Step 5. To set the new theme, you should edit the _config.php file located in your mysite folder.
The line you should edit is:
If, for example, you wish to install the wine theme from http://www.silverstripe.org/themes, after you upload and extract the theme package to the "themes" folder, you should open mysite/_config.php and change:
Here's how our Test page would look like with the "wine" theme:
(c) Copyright 2004-2013 SiteGround. All rights reserved