1. It is very important that you do not forget that some products (for example those of Themeforest and Codecanyon) in many occasions come in a double compression, that is to say, a zip file inside another zip. Check your product when downloading BEFORE INSTALLING to see if this is the case because if it is, the main zip you downloaded will not be the installable one, which will lead you to installation errors like the one in the CSS stylesheet for example. If you upload the main zip and receive an error, it is most likely because the main zip is not the installable one. Unzip the main zip and you will find the installable zip inside. 2. Remember that the Plugins, Extensions and Addons are always uploaded in the Plugins area. And the themes always in the Appearance area.