Embedded MailChimp signup code and Submit button

I’ve embedded the MailChimp signup form code to a Text widget and the Submit button is barely visible, though when I click on the button, it does take you to the Signup Form.

How and where exactly can I update the CSS file (I don’t see it in style.css – yet it says not to update this file) if I do not have the option to update the CSS code, only for Footer Scripts & Analytics in Platform Settings? I’m using the Pagelines Platform theme.

I’ve read through similar issues here in these forums and with how they resolved the issue, but I am still lost, so hoping you can help.

Thanks!