Hello,
Please try to add this code in Theme Options > Styling > Custom css:
.prefooter .mc4wp-form .et-mailchimp input[type="email"] {
color: #999;
margin: 0;
width: 100%;
background-color: #fff;
margin-bottom: 10px;
}
.prefooter .mc4wp-form .et-mailchimp input::-webkit-input-placeholder {color:#999;}
.prefooter .mc4wp-form .et-mailchimp input:-moz-placeholder {color:#999;}
.prefooter .mc4wp-form .et-mailchimp input:-ms-input-placeholder {color:#999;}
.prefooter .mc4wp-form .et-mailchimp input[type="submit"]{opacity:1;}
Regards