fixed z-index of header semicircle

brompton/sass/_extra-child-theme.scss

@@ -19,7 +19,7 @@
 		position: absolute;
 		top: -#{map-deep-get($config-global, "spacing", "vertical") + 5px};
 		width: 200px;
-		z-index: 1;
+		z-index: 3;
 	}
 }