Once you have downloaded your template, you have to upload it to your account. The files of your new style must be uploaded in a new folder under the "styles" directory. In our case, we will make a "retro" folder and upload the content in it. A correct phpBB3 style must have the following directories/files in it:
- contrib
- imageset
- template
- theme
- style.cfg
After you have uploaded the files of the new style, you should login to the Administration Control Panel. Next, click on the "Styles" tab.
Now click on the Install link next to the new template - "AZ_Retro". On the new page, select the "Make default style" option and click on the [Submit] button.
Now you can click on the "Board index" link at the top right corner of your screen and you will see the newly installed template applied to your forum!