I thought to change how SDDM appear in kde and I did but when I boot again my PC, pitch black screen appears in front of me and I’m also able to use cursor but nothing seems to appear where I can put password so that I can login into, as usual
If you are unable to login you can change the theme in a tty.
You need to edit (with sudo rights) /etc/sddm.conf.d/kde_settings.conf
Under [Theme] set Current to breeze like so: Current=breeze
Then reboot.