Downloading customized theme from wordpress

If you want easy solution, install Download Plugins and Themes from Dashboard plugin. Then you will able to download the theme. And you will able to install the theme any site you want.

Manual way:

After login to your hosting dashboard like cPanel, navigate to File Manager. Inside www directory, you will get wp-content directory, inside that, you will get themes directory. Here you will get all of your themes. You can download your theme from here.

If you can use FTP clients like Filezilla, you navigate to public_html or www folder. then navigate to wp-content/themes. You can download from there.