/etc/kde4/kdm/backgroundrc is in kdm 4:4.11.13-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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BackgroundMode=Flat
BlendBalance=100
BlendMode=NoBlending
ChangeInterval=60
Color1=0,0,200
Color2=192,192,192
CurrentWallpaper=0
LastChange=0
MinOptimizationDepth=1
MultiWallpaperMode=NoMulti
Pattern=fish
Program=
ReverseBlending=false
UseSHM=false
Wallpaper=stripes.png
WallpaperList=
WallpaperMode=Scaled
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