Breeze installs its icons in a different directory structure from other icon themes, with the result that the ECM cmake command ecm_install_icons doesn't work for Breeze icons. The only way to install an application specific Breeze icon is to hard code its location, for example "${KDE_INSTALL_ICONDIR}/breeze/actions/22/". I raised a bug against ECM about this, but not unexpectedly it has been rejected as a Breeze issue (see https://bugs.kde.org/show_bug.cgi?id=476208).
Fixing this in Breeze would obviously be a significant change for Breeze, but having a non-working ecm_install_icons function isn't really acceptable. This should ideally be fixed one way or the other in time for the KF6 release. -- David Jarvie KAlarm author, KDE developer