I have changed my display settings to my liking for the desktop, but the settings don’t seem to carry over to the login screen as well. How can I make it so my login screen uses the same display settings as the desktop?
EDIT: By display settings I meant stuff like turning off my laptop screen and making it use an external monitor instead, setting the refresh rate to 120Hz, and other stuff like that, which the login screen (SDDM) seems not to do.
Depending on the type of display-manager in use, there are different options available. See the Arch-Wiki, and from there navigate to the corresponding page of your display-manager and the corresponding display-manager-greeter settings.
sddm settings are separate from plasma settings.
Most settings are described here.
The background image or the theme selected can be modified in System Settings in plasma.
Since you didn’t clarify what you meant by “display settings”, it is a little hard to give you a better answer.
I’m not sure i understand what settings i would change on my desktop that would have anything to do with the login screen. This is something i never think about. 
Resolution, theme, hidpi settings like scaling, making the background image match the lockscreen, etc, etc
" I meant stuff like turning off my laptop screen and making it use an external monitor instead, setting the refresh rate to 120Hz, and other stuff like that "
Arandr
or more fun
i no use plasma …
guess can look in system settings,Display and Monitor,Display Configuration
I’ve never had to adjust any of these things. I set my resolution in grub to match what i set my display? The background and login screens are in settings are they not. I don’t have hi dpi issues so i don’t know how that would affect the desktop vs the login screen. I can see scaling being an issue. Glad i don’t have hardware that requires me to do that. 
So, this is supposed to allow me to set the display settings for my SDDM login screen? I remember there being a way to do this on GNOME without using any third party application, so is it just impossible on KDE without installing something first?
gdm is part of gnome.
sddm is an entirely separate application and not part of the kde project.
They way to change it’s settings is listed in the above links.
Alright, I’ll give it a whirl tomorrow and hopefully I won’t break something.
We wouldn’t need a “SirBreaker” here.
