When you remove sddm-breeze, it can leave the directory /usr/share/sddm/themes/01-breeze-fedora and a file /usr/share/sddm/themes/01-breeze-fedora/theme.conf.user behind. If no other theme for sddm is explicitly set, this broken theme is loaded anyway instead of falling back to the 02-fedora, which is part of the sddm package, leaving sddm non-functional. In the postun step of sddm-breeze, the directory /usr/share/sddm/themes/01-breeze-fedora should be removed completely, even when not empty.
Ah, in the case where you modify the theme and it creates a theme.conf.user ? I think we can account for that.
This message is a reminder that Fedora 23 is nearing its end of life. Approximately 4 (four) weeks from now Fedora will stop maintaining and issuing updates for Fedora 23. It is Fedora's policy to close all bug reports from releases that are no longer maintained. At that time this bug will be closed as EOL if it remains open with a Fedora 'version' of '23'. Package Maintainer: If you wish for this bug to remain open because you plan to fix it in a currently maintained version, simply change the 'version' to a later Fedora version. Thank you for reporting this issue and we are sorry that we were not able to fix it before Fedora 23 is end of life. If you would still like to see this bug fixed and are able to reproduce it against a later version of Fedora, you are encouraged change the 'version' to a later Fedora version prior this bug is closed as described in the policy above. Although we aim to fix as many bugs as possible during every release's lifetime, sometimes those efforts are overtaken by events. Often a more recent Fedora release includes newer upstream software that fixes bugs or makes them obsolete.
On F25 removing sddm-beeze when a theme.conf.user is present does not break sddm any longer. But a red error text is displayed on the login screen, because the theme.conf.user is still present and the invalid 01-breeze-fedora directory is used as the default theme. I still think theme.conf.user and the empty directory should be removed on uninstall, but this is now more a cosmetic problem.
Fedora 23 changed to end-of-life (EOL) status on 2016-12-20. Fedora 23 is no longer maintained, which means that it will not receive any further security or bug fix updates. As a result we are closing this bug. If you can reproduce this bug against a currently maintained version of Fedora please feel free to reopen this bug against that version. If you are unable to reopen this bug, please file a new report against the current release. If you experience problems, please add a comment to this bug. Thank you for reporting this bug and we are sorry it could not be fixed.