Hello everyone,
I would like to customize my login and lock screen. I understand that the configuration scripts of SDDM are under /etc/sddm. However, this directory does not contain any theme specific files. So, if I just to want to customize the default theme (for example to change the font color, or something similar), I need to modify the /usr/share/sddm/themes/<theme> files.
BUT, this will be overwritten on the next upgrade! So, how do I customize the theme without changing the original files?

As for the lock screen. I know that the customized lock screen per user is configured in the ~/.config/kscreenlockerrc file.
But, how can I change things like the fonts, the clock settings or the position for example of the items? Any documentation on that would be of great help.

Both the login screen (SDDM) and the plasma lock screen are using the Qml:

Qml wiki: https://en.wikipedia.org/wiki/QML
Qt Qml: http://doc.qt.io/qt-5/qtqml-index.html
SDDM theming: https://github.com/sddm/sddm/wiki/Theming

how do I customize the theme without changing the original files?

You could copy the theme under new name.

Many thanks for the links.
Then, I need to file a bug report. It doesn't make any sense (especially for the lock screen) to create new themes in order to fine tune an existing one!
But, anyway, the point is that I can do what I want now.

Few points of the lock screen colors and customizing: viewtopic.php?f=14&t=151172

i.e.

With the Plasma 5.12 it is possible to add lock screen configuration to the LnF theme. The Plasma 5.12.2 or later is needed to show the configuration options.

The configuration options can be added by editing the Look and Feel theme lock screen config.xml and config.qml files...