Unzip the style you have downloaded. Upload the contents found within the upload/ folder to your sites root directory.
Once all the files have been uploaded log into your AdminCP and navigate to:
Extensions >> Theme >> Import Styles
Look for the table:
Manual Install
Here is where you can find all the styles that can manually be installed. Click on the "Install" to install a style.
Unzip the style you have downloaded. Upload the contents found within the upload/ folder to your sites root directory.
Once all the files have been uploaded log into your AdminCP and navigate to:
Tools >> Maintenance >> Cache Manager
Click on "Clear All" to clear your sites cache.