How to install a new phpBB3 template
For the purpose of this tutorial we will install the "Avalon" template on our forum. You can find a big database of styles at the official phpBB website, as well as in the paid phpBB templates collection by TemplateMonster.com.
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 the "avalon" folder and upload the content in it.
After you have uploaded the files of the new style, you should log in the Administration Control Panel. Next, click on the Styles tab.
Now click on the Install link next to the new template - "Avalon". On the new page, select the Make default style option and click on the Submit button.
Follow the Board index link at the top right corner of your screen and you will see the newly installed template applied to your forum!