Add proof of concept gradient only for middle of navbar

Vertically, with hard stops to switch to solid colors right in the middle.  Pretty cool.

web/themes/custom/geofresco/src/global/overrides.scss

@@ -15,6 +15,18 @@
 }
 */
 
+.navbar {
+  background: linear-gradient(
+    white,
+    white 20px,
+    lightgreen 20px,
+    green 40px,
+    green 60px,
+    white 60px,
+    white
+  );
+}
+
 .footer {
   background-image: url('/themes/custom/geofresco/images/grassland-wark-forest.jpg');
   background-position: center;