This task tracks our design process for the 5.16 lock screen theme redesign.
I will repeat the pertinent reasons from D16031#392692:
> With the original design, the elements all float above a blue background. But the blue background was ugly and unpopular and people replaced it with wallpaper. This looked pretty but introduced contrast problems. So we put the wallpaper there by default and blurred it to ensure contrast, but that defeated the purpose of adding the wallpaper since you couldn't actually see it! So now we get user complaints that they can't see the wallpaper on the lock screen and that it looks like a dark muddy mess. In this patch I removed the blur and tried to solve the contrast problems with a combination of drop shadows, new icons with built-in backgrounds, and a bottom bar, but the result never felt cohesive and our designers kept trying to redesign it over and over again--a sign that it still wasn't right. It started to feel like a game of whack-a-mole.
> The root of the problem is that with this basic design, all the different elements want to hover over the background, unconstrained by a box or frame. This requires tight control over the background which in practice doesn't work since it's user-customizable and limits our own design choices (e.g. we could never use a light-colored wallpaper since then the text and icons would be illegible).
> What I think we need to is come up with a new design that accommodates arbitrary backgrounds from the start, probably by putting all the UI elements in some kind of box, like the Plasma ≤ 5.7 theme, Elarun theme, Maldives theme, etc. And most importantly, they should feel good in a box or frame. It shouldn't feel like they're uncomfortable together, like they want to escape.
With that in mind, let's come up with a timeless, permanent design for 5.16 that nobody will want to redesign!
== Design goals
1. Seamlessly handles the most common use case of a single user machine
2. Seamlessly handles the second-most-common use case of a 10+ user shared machine in an institutional setting. Neither one should be sacrificed for the other!
3. It's 100% obvious which user will be logged in to; it should be impossible to accidentally try to log in as the wrong user
4. Looks good with any wallpaper, color, video, animation, etc as the background by putting UI elements on some kind of box/frame/etc
5. Has some visual continuity with the rest of Plasma
6. Does not lose any features compared to the current one (i.e. still has a session chooser, keyboard chooser, etc)