Created attachment 180808 [details] Example for comparison When using the Breeze Dark theme, the SDDM login screen’s password box and login button defaults to a white background. This differs from KDE lock screen,disrupting system visual consistency.
Currently you need to sync your Plasma settings to SDDM to make it pick up visual changes made in your user account. You can do this in System Settings > Colors & Themes > Login Screen (SDDM) > Apply Plasma Settings… In the future we may be able to make this automatic on systems with only a single user account.