Make sure you are in back office of PrestaShop and had disabled performance setting.
Click on Preference > Themes
On top green color horizontal bar, click on “Add new theme” button.
From “Import from your computer“ fieldset click on “Add File” button to install zip file from your
computer, which you have already download installable theme zip file.
Once you imported zip file you need to active theme and proceed step by step with default
Do not forget to regenerate all thumbnails once you install themes.