tde-packaging-slackware/Apps/kvkbd/README

The many enhancements include setting key and/or background colours, and displaying level 3 & 4 characters where the xkb/symbols definitions include using the AltGr key [include "level3(ralt_switch)"].

Build-time options to be hard-coded into the app are:

[1] Use each Win key
    a] as a modifier key
       The default use for the \Zb\Z6LWin\Zn and \Zb\Z6RWin\Zn keys on the \Zb\Z6Kvkbd\Zn keyboard is set as modifier keys to map a number of characters or functions and to 'lock' for the next key click - this is to simulate holding down the key on a physical keyboard.

 or b] for a character set with xmodmap
       Alternatively, using xmodmap, each key can map to a character set by a user - this will generate a keycode on a single click.

[2] Alternative text on the num pad keys.

For example:
|Ho|
|me| can be replaced with |H←|, which is a better fit on the keys.

[3] Small icons are a better fit on the buttons if the keyboard is to be used at the default size.

[4] Show blank keys where AltGr doesn't produce a character.

Usually, not all keys have 3rd and 4th level characters assigned.
Select this option to see only those characters which AltGr or AltGr+Shift generate.