In localhost there on your own machine, just use Explorer or whatever file manager you might have and either download or copy the theme into your /wp-content/themes/ folder and it will appear at the Dashboard.
I recomend you install the wordpress as usual in internet and protect the full site with htpassword til you finish the work.
The problem is that your localhost server does not have an FTP server to access the file system.
Rather than placing the files yourself (which works fine), you can evade this error by adding this to your wp-config.php file:
define('FS_METHOD', 'direct');
This will also let you install plugins.
I tried to upload the premium theme via appearance but it says:
Are you sure you want to do this?
Please try again.
That message is frequently an indicator that the archive you’re trying to install is larger than your maximum allowed upload size. (usually ~2MB limit by default). Premium theme packages are often fairly large, and contain much more than just the theme files.
Read the installation documentation that came with your theme.
