improve black theme

scripts/black_theme.json

 {
-    "ASP": "\nobj_property(edge,color,white).\nobj_property(edge,fontcolor,white).\nobj_property(graph,bgcolor,black).\n",
+    "ASP": "\nobj_property(edge,color,white).\nobj_property(edge,fontcolor,white).\nobj_property(graph,bgcolor,black).\nobj_property(node,color,white).\nobj_property(node,fontcolor,white).",
     "description": "Draw with a black background",
     "name": "black theme",
     "tags": ["theme"],