This theme is an addition to elementary OS's default icon theme. It contains icons for several third party applications that resemble elementary OS's style.
E: The repository 'http://ppa.launchpad.net/cybre/elementaryplus/ubuntu bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
Err:17 http://ppa.launchpad.net/cybre/elementaryplus/ubuntu bionic Release
404 Not Found [IP: 91.189.95.83 80]
E: The repository 'http://ppa.launchpad.net/cybre/elementaryplus/ubuntu bionic Release' does not have a Release file. N: Updating from such a repository can't be done securely, and is therefore disabled by default.