MailChimp submit button barely visible, but working…

Hi,

I am using the PageLines Platform theme (not Pro version) and have embedded the MailChimp signup form code to a Text widget. And the Submit button is barely visible and grayed out, 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’ve read through similar issues here in these forums and with how they resolved the issue, but I am a real beginner with CSS, so hoping you can help.

Thanks!