In the previous versions of OpenCart, it can be difficult to install a template but this has certainly eased out in the later versions of OpenCart.
The step described below to install openCart template is based on openCart version 1.5x.
Here are things you need to know:
Make sure you understand the version that the template you want to install supports
All templates are saved in the directory : /catalog/view/theme/
So if your website root directory is /public_html/store, the the files will be saved in /public_html/store/catalog/view/theme/
Step 2: Extract the template using your favorite unzip software
In the example above, you can see that the folder contains the opencart version.
Click on OpenCart1.5.4.x to expand the folder.
Then extract the files to the theme folder.
In some cases as below, you only need to unzip the the entire folder to the theme directory on your server.
Step 3: Go to the admin screen to enable the theme
If everything goes well, the theme should show up in the dropdown list as follows:
Login to the admin screen
Click on System->settings
The page below displays. Then click on edit