How can I change the appearance of the form while it is being submitted

When submitting my form, I'd like to be able to add a 'working' indicator, and maybe disable the submit button so that the user knows the form is being submitted.

I had expected BoltForms to add a class to the `` or submit `` elements, which could be targeted with CSS, but nothing actually happens until the AJAX request is completed, or the page is redirected.

I've looked at the `htmlPreSubmit` and `htmlPostSubmit` options, but these only add static HTML on the page with and without the form respectively.

Is there currently any way to achieve this?

I'm using BoltForms 4.2.3 and Bolt 3.5.