Your current user's color scheme and font preferences can be synced to the SDDM login screen in the upcoming Plasma 5.17, which should take care of part of this request.

As for the blur, that's tricky. The first problem is that the SDDM KCM doesn't expose any theme-specific options, so there's no way for the default Breeze SDDM theme to offer a way to turn off the blur. Even if we could, that would offer the possibility of making the buttons and their text unreadable with the current design.

You might ask, then, "well, why don't you just change the design, duh?"

This is easier said then done! :) The current design is the result of a lot of refinement and discussion. In the Plasma 5.16 timeframe we had multiple discussions regarding alternative designs that would not require a blurred background. The problem is that we allow users to set arbitrary wallpapers here, so we need some way to make sure that the things on top of the wallpaper remain visible, no matter what it is. There are only a few options:
1. Blur and darken the wallpaper
2. Put outlines or shadows behind everything
2. Put everything inside/on top of a frame or window that's above the wallpaper

#1 Is the status quo.
#2 Is what we do this for Desktop icons, but we could not find a way to make this work serviceably for the lock screen's use case. The possibilities for the wallpaper were too broad for us to be comfortable implementing it.
#3 is a complete redesign.

Personally I was pushing for #3, but like so many things in life (and especially with a large and complex project that involves other people), sometimes you don't get exactly what you want and that's what happened here. So we stuck with #1 but reduced the darkness and blurriness of the blur. This reduced the number of complaints, but I had a feeling that some people would still not be satisfied.

Given that there is no remaining interest or energy in touching the login screen for quite some time (the discussions were exhausting), the only path forward I see here is making the blur completely disable-able, which would require changes in both the theme and the SDDM KCM to display per-theme user-visible options.

Moving there for now.

Thank you for the thorough answer and for your time!

A last word before moving to another login theme.

I've read the discussions myself and I have come to the same conclusion. The large range of ideas and opinions have led me to the request, the settings made available would be solution because they would let people tune it according to the chosen image. I know it's possible. The only thing it keep me from implementing it is the lack of knowledge in QT, as you already know.
Importing the theme is something I am sure will make more trouble. BTW, whose theme? There is no current user at the login time... But I'm sure I am missing something...

The feature in Plasma 5.17 allows a user with admin rights to sync their user settings to SDDM. It's not automatic, it's manual. In the future we hope to optionally make it automatic for the common case of a single-user system where the user has admin rights.

Have you checked out https://github.com/flipwise/sddm-breeze-514?

There's a bunch of options you can edit in theme.conf; most notably you can turn off blur. 

If there's an option missing, you can have a look at how I added options and you'll see it's easy. Testing is also fast since you can just do: https://wiki.archlinux.org/index.php/SDDM#Testing_(Previewing)_a_Theme

I am a software developer, I don't mind to do it this way, but it's pretty unfriendly for everyone else. I must admit, I am pretty baffled by the QT framework. I have some little experience while trying to get the wallpapers sorting to work, but even today I don't understand how the job is done while I see the path is served to the sorting object, yet the filename is sorted.
So, for me, it's a bit difficult to use QT, despite I am willing.

Back to this one, do the theme config file is overridden at some point - like upgrading, that would make me re-do the config again and again (I am using a rolling release distro)? That would make things really pointless from the usability angle..

If you update an SDDM theme yes it's going to get overridden.

But if you keep a fork of your own it's no problem.

As for exposing more options in the KCM, it's tricky. We can't just add an option to disable blur because then we need to add a bunch of workarounds to ensure legiblity. And that might mean adding another set of options, which then creates clutter and defeats the purpose of having SDDM themes and the syncing feature implemented in 5.17.

I could be convinced of having only having one option to disable blur & wallpaper fader, which could then trigger all the needed workarounds to show up as well. The problem with that is we're then adding an option to the KCM that pertains solely to the Breeze theme :/

What is this you are talking about?
Despite it might sound rude, none is intended: is this a bad design?
I am totally newbie to the internals of this, so I ask: What is the issue, actually?
Is it missing some generic approach? Like reading some config and passing the options to the UI and back to the implementation? Isn't that pluggable like an applet, like the wallpapers various settings, depending on the chosen plug-in, like slideshow, or colour and so on?

My suggestion was that the KCM gains the ability to read some metadata in the theme files itself to determine which options to show to the user. Then we wouldn't have to hardcode anything in the KCM and themes could expose arbitrary settings to the user (or maybe just checkboxes for simplicity's sake).

And yeah, my idea was that the proposed "disable blur" setting would automatically enable all the contrast and readability aids.

"Was"...?

Well okay, "is". :)

"but"...