override normal flowchart arrowhead to allow css styling

src/less/forest/flow.less

@@ -12,6 +12,10 @@ color:#333
   stroke-width: 1px;
 }
 
+.arrowheadPath {
+  fill: @arrowheadColor;
+}
+
 .edgePath .path {
   stroke: @lineColor;
   stroke-width: 1.5px;

src/less/forest/variables.less

@@ -4,6 +4,7 @@
 @lineColor: green;
 @border1: #13540c;
 @border2: #6eaa49;
+@arrowheadColor: green;
 
 /* Flowchart variables */
 @nodeBkg:@mainBkg;