Customizing Jetpack email Suscription widget via CSS

Hi,
I would suggest that you use Custom CSS to customize the look of that button. Try adding your own styles to your theme stylesheet, or to your custom CSS editor under Appearance > Edit CSS in your dashboard. You can target the submi button with the following CSS:

.jetpack_subscription_widget input[type="submit"] {
     /* Your custom CSS rules here */
}
/*I think it would look better if you were to use a simple background, like so*/
#subscribe-blog-blog_subscription-2 input[type="submit"] {
       background: #666;
       border: none;
       color: #fff;
       height: 32px;
       padding: 0 15px;
}

Or try the following CSS:

.jetpack_subscription_widget input[type=text], .jetpack_subscription_widget input[type=email] {
color: #333;
padding: 7px !important;
width: 95% !important;
border: 0;
}
.jetpack_subscription_widget input[type="submit"] {
background: BUTTON BACKGROUND;
color: white;
margin: 0;
width: 100% !important;
line-height: normal !important;
padding: 6px !important;
height: auto;
border: none;
box-shadow: none;
text-shadow: none;
}
.jetpack_subscription_widget input[type="submit"]:hover {
background: BUTTON HOVER BACKGROUND;
}

Note: The Hueman theme not a product from our team.
Hope this helps !