How do I add CSS to my `mailchimp` popup form?

Question

I am using mailchimps popup subscribe form. I have styled the content in mailchimp as far as I can. I want to add some more CSS to the popup, I have tried adding CSS to the CSS sheet already loading in my site yet the form will not pick it up in the popup even with !important etc.

The code from mailchimp is:

<script type="text/javascript" src="//s3.amazonaws.com/downloads.mailchimp.com/js/signup-forms/popup/embed.js" data-dojo-config="usePlainJson: true, isDebug: false"></script>
<script type="text/javascript">require(["mojo/signup-forms/Loader"], function(L) { L.start({"baseUrl":"mc.us8.list-manage.com","uuid":"731f7bb19522a201007a53699","lid":"138afe0d12"}) })</script>

Now, I have tried also adding inline styling in my page for a test, example:

<style>
    #mc-EMAIL {
        border:6px solid pink !important;
    }
</style>

I have noticed my styles for the mailchimp are coming from this however mailchimp say I do not have access to change this CSS file.

My overall question which I am sure someone else must have faced before me: