Updated: October 2, 2024
I believe, in all my years of writing on Linux and covering Plasma, I've only briefly touched on the customization of the login and lock screens. But as you well know, this desktop environment is super-customizable, so you can do pretty much anything you like. Well, just recently, I encountered a scenario that prompted me to take a more profound look at these two elements, and tweak them to my liking.
My 2014 IdeaPad Y50-70 laptop, which runs Kubuntu 24.04 on it, has a 4K screen. This means, even if you configure desktop scaling, the login screen will, by default, look weird. Tiny, out of place. Add the bland wallpaper used in Kubuntu 24.04 into the mix, and you're really not starting your session on a positive note. The lock screen does respect your scaling, but it does not use your wallpaper. Let us rectify all these tiny issues, so we have a spotless desktop. Begin, we shall.
Some more background information ...
Just before we touch on the technical elements, let me tell you a bit more about my setup. The old laptop now has an SSD in it, which grants it speed and vitality. Kubuntu 24.04 started weakly, but has improved since. My machine is quite interesting, because it has a 4K screen. This wasn't that common when it was released, back in 2014, and the UHD resolution did create a bunch of problems vis-a-vis Linux over the years.
Now, I've written guides on how to manually fix the GRUB resolution and SDDM resolution on 4K screens. But these are nerdy solutions, and we should avoid them as much as possible. Indeed, the goal here is to show you how to achieve pretty, slick results without any command-line tweaking. Well, now that you have all the relevant details, we can begin with the customization. Avast. Or something.
Customize login screen
Open Settings > Workspace > Startup and Shutdown > Login Screen. In the right pane, there will be a number of preinstalled login themes available, including your current (default) one. Click on Apply Plasma Settings at the bottom of the screen. This will use all your current system tweaks as the baseline for the login screen. Click Apply. Next, click on the image icon of your selected theme, select a new background image, and confirm your change.
As an aside, I noticed in Kubuntu 24.04, the background image selection step takes a long time, more than a minute for the actual buttons to show in the overlay windows. This seems to be a bug, but eventually you will be able to make the necessary changes.
Your login screen should now fully match your desktop, including the fonts, scaling, cursor, and whatnot.
Customize lock screen
The login screen tweaking doesn't handle screen locking. You will need to do that yourself. The path yonder is via Settings > Workspace > Workspace Behavior > Screen Locking. Here, you have lots of options, but the most interesting part is under Appearance. Select any image or wallpaper you have - those you've manually added for your desktop wallpaper will also be in the list. Confirm your change, and you're golden.
Optional: Customize splash screen
The login screen process also does not cover the splash animation - the step between entering your user credentials and the desktop session loading. Normally, you see the KDE cogwheel logo animation, but you can customize that one, too. In fact, if you decide to use no splash, you might even speed up your login a bit. If you have set your system to log in automatically, this step is redundant. Settings > Appearance > Splash Screen. You can also customize the boot splash screen if you like, w00t.
Conclusion
And there you go, now you have a slick and consistent desktop experience, where all the different elements match, in every aspect. It's a great thing that Plasma lets you customize the login screen automagically, but it would be even nicer if the lock screen and splash animation were all part of the same bundle. There also seems to be a bug with the image selection for the login, but I guess that ought to be ironed out, and shouldn't bother you too much, if you encounter it.
The Plasma desktop is highly versatile, elegant, and you can customize everything. Best part? Don't worry if you mess up. You can always go back to defaults, and reset your configuration. Every settings menu has that option, so you can just undo all your work. If you select the wrong images, fonts, change your mind, whatever, you can easily restore the baseline and start fresh. That's another superb thing about this fine desktop. Well, on that note, let us fade into the sunset. Or something.
Cheers.