this is easy
to be root and edit
/usr/bin/startkde
find the line "xsetroot -solid" and change the color value
if test -z "$XDM_MANAGED" || echo "$XDM_MANAGED" | grep ",auto" > /dev/null; then
# xsetroot -solid "#5477A0"
xsetroot -solid "#000000"
fi
this example changes the 'blue' background to 'black'
carlos