In many places (in terminal) when an app is launched, it complains about missing icon themes that were installed in the past. Ironically, this is always in case of themes installed by… package manager. Those added manually have never this problem.
So for example:
$ QT_STYLE_OVERRIDE=Breeze libreoffice --calc
Icon theme "Papirus" not found.
Icon theme "Yaru" not found.
Where is this info stored? I would gladly remove it manually.
Check index.theme in your current icon theme (in ~/local/share/icons or /usr/share/icons). You most likely have those nonexistent icon themes mentioned in Inherits= line. Remove them and message will disappear