After purchasing this theme, please download the package from the Themeforest. Login to your themeforest account and click Download. Here you will find the items which are you have purchased.
Then click the Download > Main File(s) button next to Designthemes theme and save the package on your computer.
After downloading it from Themeforest,unpack the whole package. In that package, you can find things like documentation.zip, spalab.zip, spalab-child.zip, and revolutionslider-export.zip files.
If you try to install the wrong files you will get a missing styles.css file error. This is the most common error, when you are trying to install incorrect/whole packages.
Update WordPress : We highly recommend you to update your WordPress to the latest stable version 4.0 or higher. This will help you to ensure that you are able to utilize all of the latest features of WordPress.