How to upload a Magento theme?
Once you pick a theme and open its details page click on the Get Extension Key button, agree to the extension license agreement and click again on the same button.
The extension key will be visualised. For the current example the extension key is:
magento-core/Interface_Frontend_Default_Modern
Select and copy it.
Open the Magento Admin Area->System->Magento Connect->Magento Connect Manager. Enter again the admin login details and paste the extension key in the Paste extension key to install field. Click on the Install button.
Once the installation is completed return to the Magento Admin Area and open System->Configuration->Design->Themes.
Enter the theme name in the Default field and click on the Save Config button.
The front page might not load correctly. The reason could be the number of the columns in the new theme's home page layout.
You can fix this through the Magento Admin Area->CMS->Manage Pages. You will find two home pages. One is with 2 columns layout and the other is with one. Disable the first one and enable the second by clicking on their status links.
Now the front page will appear correctly with the new theme.
