How to install themes in PrestaShop
You can install new themes in PrestaShop either manually or by using the theme installer that comes prepackaged as a module with the PrestaShop installation pack.
You can search for new themes in the addons section of the official PrestaShop site.
To use the automatic theme installer, first you have to install it (it's uninstalled by default). Log in to the backend of your store, click on the Modules tab, then in the Administration module group find the module Import/export a theme and click on its Install button.
After it's installed click on the Configure button under the module's name. You'll be presented with three ways to upload the new theme. Choose the method which you prefer and click on the Next button. On the next page confirm that you want to install the theme by clicking on the Next button. On the following page, if the theme comes prepackaged with modules, select whether you want to install these modules, and you also have to select whether you want to use the theme's modules and images configuration. Click on Next to continue. On the next screen you'll see a list of the actions performed by the installer; click on Finish to complete the process.
To make the new theme the default one, go to the Themes sub-tab of the Modules tab, select the theme and click on Save. Keep in mind that depending on the new theme and on the previous one, you may have to disable/uninstall some modules, and possibly change the position of others.
To install a theme manually, download the archive to your local computer and unzip it. The unzipped folder should contain at least a folder with the theme's files and directories. Upload that folder to the themes folder in the root PrestaShop directory on your hosting account. Optionally, depending on the theme, the unzipped folder on your local computer may also contain a modules folder. In this case, you also have to upload the folders that are inside the modules folder to the modules folder in the root PrestaShop directory on your account.
To switch to the new theme, log in to the backend of your store, click on the Modules tab and then on the Themes sub-tab. On the page that opens find the Themes section, mark the button for the new theme and click on Save. If you uploaded modules for that theme, go back to the Modules tab page of your store's admin panel, find the new modules you uploaded earlier and install them. If the unzipped theme archive on your local computer contains some installation instructions, check them to see whether there's something additional you have to do.
For some more details and screenshots check out the PrestaShop Themes Tutorial.