Dashboard Background Image

Hi,
I try to modify dashboard layout....
Is it possible to have a backgrund image over which

several table views float.
Idea is for example to have a factory image as background,a

and the individual production lines are shown as table views

floating above.
I tried to use CSS (not faniliar with CSS yet), but failed

already in setting a new body background color...
body {

background-color: lightblue;

}
as custom CSS (for example for having another background color)

together with checking {this page has custom settings }

does work in IDE but on page only flashes shortly blue, then

goes back to standard color
Any suggestions?
Kind Greetings,
ckranich