I had the exact same issue at first, with the same theme! It turns out that the fix is actually really easy and the explanation is readily available in the documentation videos supplied.
You’re not supposed to install the entire initial zip folder that you get when you press download from envato (themeforest-166299-king-size-fullscreen-background-wordpress-theme.zip). Instead, unzip it and then go through the folders like below:
> KingSize WordPress v.5.0.6 > 1. Version 5.0.6 (NEW)
Once you reach the folder above, you should see a few more zip files and folders, but the important ones are kingsize.zip and the corresponding unzipped folder. Install via wordpress using the kingsize.zip folder!
If that doesn’t work (for me it didn’t work because my connection kept resetting), just use FTP and pull the already unzipped folder into your wp-content/themes folder, and that will get the job done. Hope that helps!
