Fix handwritten background style not showing

Author: jeanmarc

dist/plantuml-stdlib/stdlib/archimate/themes/shared_style.puml

@@ -9,7 +9,7 @@ skinparam StereotypeAlignment right
 skinparam lineType $ARCH_LINETYPE
 
 ' fallback values if theme does not define it
-!global $ARCH_DEFAULT_BACKGROUND = "#white"
+!global $ARCH_DEFAULT_BACKGROUND ?= "#white"
 !global $ARCH_DEFAULT_FONT ?= "Verdana"
 !global $ARCH_DEFAULT_FONTSIZE ?= 12
 !global $ARCH_HANDWRITTEN_BACKGROUND ?= "#eeebdc"