Highlight a targeted line in source block

When using refs to create links to specific lines in code blocks, use the
‘:target’ pseudo-class to highlight the background.

src/less/main.less

@@ -75,3 +75,7 @@ pre.src {
   line-height: 1;
   padding: 0;
 }
+
+.org-src-container span:target {
+  background-color: #1b214a;
+}