update codemirror formatting-header opacity

src/client/styles/scss/_override-codemirror.scss

@@ -8,7 +8,7 @@
     .cm-formatting-header {
       font-style: italic;
       font-weight: bold;
-      opacity: 0.3;
+      opacity: 0.5;
     }
 
     .cm-header-1 {