After changing the SDDM theme I can’t log in. I think it’s because the new SDDM forces the first letter in the username to be capital or it might just be an purely visual thing

If all you did was to change your sddm theme: since you cannot revert your changes in the Gui, because you cannot log in you can switch to a TTY CTRLALTF2 and change /etc/sddm.conf.d/kde_settings.conf in the [Theme] section set the theme back to the original (breeze): Current=breeze instead …