Je creuse encore.
Le fichier de conf. de Kdm (/etc/kde4/kdm/kdmrc) semble correct.
Un extrait :
[X-*-Greeter]
AntiAliasing=false
ColorScheme=Oxygen
FaceSource=AdminOnly
FailFont=Sans Serif,10,-1,5,75,0,0,0,0,0
GUIStyle=Oxygen
GreetFont=Serif,20,-1,5,50,0,0,0,0,0
GreetString=Bienvenue dans %s sur %n\s
GreeterPos=50,50
HiddenUsers=
Language=fr
NumLock=On
LogoArea=Logo
LogoPixmap=/usr/share/kde4/apps/kdm/pics/kdelogo.png
MaxShowUID=29999
MinShowUID=1000
Preloader=/usr/bin/preloadkde
SelectedUsers=
ShowUsers=NotHidden
SortUsers=true
StdFont=Sans Serif,10,-1,5,50,0,0,0,0,0
Theme=/usr/share/kde4/apps/kdm/themes/joy
UseBackground=true
UseTheme=true
UserCompletion=false
UserList=true
Je sèche toujours.