I played around a little bit with some themes.
Unfortunately by now the login screen (where you put in your password)
just shows grey dots instead of blacks.
My eyes aren´t the best, so I would like to restore it.
Does anyone know where I can set that back to black?
I don’t know, but you might be content just by switching to a different login manager. XDM, GDM, SDDM, LightDM are the customary choices you should be able to find in YaST, if all are installed. The other switching option is the command update-alternatives --config default-displaymanager.
Someone else might be better able to help if you tell us which DE and DM you are using now.