You can install another lock screen theme, if you like. There are plenty to choose from over at store.kde.org, but make sure that you pick one that is…
still maintained by its developer; and…
compatible with the current versions of KDE Plasma, the KDE Frameworks and Qt5.
Edit: Upon second inspection, you cannot change the lock screen’s theme, but you can change what it does, e.g. show a wallpaper, a slideshow, et al. And for this, additional plugins are available as well ─ I think you’ll even find some in the AUR ─ of which some should already be installed on your system.
Also, if you choose a different Plasma theme ─ i.e. the theme for the panels and the widgets ─ then this will affect the look & feel of the lock screen as well.
If you navigate to…
System Settings → Workspace → Workspace Behavior → Screen Locking
… you will see a button for configuring the appearance of the lock screen. If you click that, you’ll see a setting labeled “Wallpaper Type”. It has several options you can choose from.
If either of these two commands works ─ you can test this by way of a terminal window ─ then you could create a desktop shortcut with the working command in its execute field, and then add this shortcut to your desktop and/or to a panel.
I don’t like the look of it. I prefer how the lock screen and greeter screen look on gnome. But I do like how KDE looks like overall and the ease of customization.
Yes, the commands work, I’ve tried them before.
That would only solve half of the problems, though. What about when the computer is set to sleep? I’d need to replace the way kde handles the lockscreen altogether, otherwise it’s not really fixing the problem. I’m not a linux noob btw, but I’ve never dug this deep into the system, as there was never a need.
I’m not really sure how to proceed.