add non-white bg to marker symbols mock

- so that -open symbols are noticeabily NOT filled in white

Author: etpinard

test/image/baselines/marker_symbols.png

@@ -500,6 +500,7 @@
       "autorange": "reversed"
     },
     "showlegend": false,
+    "plot_bgcolor": "#d3d3d3",
     "hovermode": "closest"
   }
 }