Error message styling improvements

Hi!

First of all, thanks for making this plugin publicly available: so far the shortcode has been a great help to integrate a SendInBlue signup form into our client’s website.
I’m trying to customize the way the form box is styled – specifically the error/validation messages – and it looks as if there are not many options available (yet) for developers to filter or modify the markup responsible for the messages.
The relevant code lies in sendinblue.php, specifically the generate_form_box() function.

There is a CSS spec that _could_ be helpful in the future, but this doesn’t work yet:
.sib_msg_disp:has(.sib-alert-message-warning)
this would allow us to target this element and the ones around it using sibling selectors etc.

So for easier CSS targeting, it would be helpful to have:

– apply_filters() for (parts of) the output
– css classes that are added to the form or individual fields based on the validation state, like ‘hasError’ ‘isValid’ etcetera. This should be fairly easy to achieve by adding it to the JS.

Is this something you will be working on in the future? Many thanks!