LightDM

LightDM is a cross-desktop display manager whose aim is to be the standard display manager for the X server.

The key features (as listed by upstream) are:
 * A well-defined greeter API allowing multiple GUIs
 * Support for all display manager use cases, with plugins where appropriate
 * Low code complexity
 * Fast performance

Installation
Install :

Configuration
The configuration file for LightDM itself is:

KDE
The LightDM KDE greeter also provides a KCM (KDE Control Module), which appears in the System Settings under System Administration → Login Screen (LightDM).

Due to LightDM currently assuming GTK+ greeter as the default, if the KDE greeter is requested, it needs to be specified in the file:

RazorQt
The RazorQt greeter currently does not offer any configuration options.

OpenRC
Set LightDM as the default display manager:

To start LightDM on boot, add dbus & xdm to the default runlevel:

To start LightDM now:

systemd
To start LightDM on boot:

To start LightDM now: