:~$ sudo apt update
Hit:1 http://security.ubuntu.com/ubuntu xenial-security InRelease
Ign:2 http://ppa.launchpad.net/bsundman/themes/ubuntu xenial InRelease         
Hit:3 http://ro.archive.ubuntu.com/ubuntu xenial InRelease                     
Hit:4 http://ppa.launchpad.net/numix/ppa/ubuntu xenial InRelease               
Hit:5 http://ppa.launchpad.net/openjdk-r/ppa/ubuntu xenial InRelease           
Hit:6 http://ro.archive.ubuntu.com/ubuntu xenial-updates InRelease       
Err:7 http://ppa.launchpad.net/bsundman/themes/ubuntu xenial Release           
  404  Not Found
Hit:8 http://ro.archive.ubuntu.com/ubuntu xenial-backports InRelease           
Reading package lists... Done                      
E: The repository 'http://ppa.launchpad.net/bsundman/themes/ubuntu xenial Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.