heatmap: fix black selection area

public/sass/components/_panel_heatmap.scss

@@ -47,6 +47,13 @@
   }
 }
 
+.heatmap-selection {
+  stroke-width: 1;
+  opacity: 0.3;
+  fill: #828282;
+  stroke: darken($red,15%);
+}
+
 .heatmap-legend-wrapper {
   @include clearfix();
   margin: 0 $spacer;