Add style for the window selection

src/style/genome-browser.css

@@ -7,4 +7,12 @@ svg.active * {
 }
 .gene {
   cursor: pointer;
+}
+
+
+.genome-axis-window {
+  fill: lightgrey;
+  fill-opacity: 0.7;
+  stroke: lightslategray;
+  rx: 2;
 }
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