Add highlighting style for placeholders

This is in accordance with Google's developer documentation style guide.

static/main.css

@@ -52,6 +52,10 @@ code {
     border-radius: 2.5px;
 }
 
+var {
+    color: #ba1a1a;
+}
+
 #site-menu ul {
     display: flex;
     flex-wrap: nowrap;