[fix] simple theme: set img background color to wallpaper's color

The *background color* of an transparent image is the color of the color of
wallpaper on which all images are shown on.

Set variable `--color-result-image-background` and `--color-base-background` to
the same value.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>