CSS: Style .form-help

sass/_core.scss

@@ -534,6 +534,11 @@ body.converse-fullscreen {
         display: block !important;
     }
 
+    .form-help {
+        color: var(--subdued-color);
+        font-size: 90%;
+    }
+
     .form-control--labeled {
         margin-top: 0.5em;
     }