CSS: Remove error highlight

(cherry picked from commit c8d7e0a4ee974814e7ef9b32968ba2451cf602b8)

_static/css/custom.css

@@ -544,6 +544,11 @@ code,
 
 /* Syntax highlighting */
 
+/* Remove default red boxes around Pygments errors */
+.highlight .err {
+    border: none;
+}
+
 .highlight {
     background-color: var(--highlight-background-color);
 }