Step 1 - Get a latest version of zncstrap. Install Git / Git Core on your Linux machine or alternativly, install and use wget or aget tool; or the good ole way via FTP/sFTP.
Step 2 - We strongly advice you to make backups of your current files/folders before you proceed to install zncstrap. Once you have your backups stored elsewhere, you can proceed to delete/remove the default theme(s).