A very simple theme without any fuss, shutdown and reboot buttons with an input, the input doesn't have any borders or anything, you just type in the password and enter.
If you want to integrate this with nitrogen to get your login background to sync with your desktop background, put this script in your /usr/share/sddm/scripts/Xsetup:
rm /usr/share/sddm/themes/mahdi/background-link
cp $(cat /home/mahdi/.config/nitrogen/bg-saved.cfg | head -n 2 | tail -n 1 | sed 's/file=//') /usr/share/sddm/themes/mahdi/background-linkIf you want to change the background, change the background-link to something else using:
sudo ln your-desired-background.jpg /usr/share/sddm/themes/mahdi/background-link