Implement --background-color (#1508)

Co-authored-by: Rangi42 <sylvie.oukaour+rangi42@gmail.com>

contrib/bash_compl/_rgbgfx.bash

@@ -19,6 +19,7 @@ _rgbgfx_completions() {
 		[Z]="columns:normal"
 		[a]="attr-map:glob-*.attrmap"
 		[A]="auto-attr-map:normal"
+		[B]="background-color:unk"
 		[b]="base-tiles:unk"
 		[c]="colors:unk"
 		[d]="depth:unk"