Hello,
You already have custom CSS applied for this form under Appearance > Customize > Additional CSS.
In your code (around line 821), the border-bottom property is being applied to the entire newsletter container, which is why the gap appears. To fix this, I recommend applying the border-bottom specifically to the email input field instead, for example:
.newsletter-form input[type="email"] {
border-bottom: 1px solid #000;
}
Best Regards,