Yes I show you with instructions.

1. Edit the class of the button in the Mailchimp form code to what you see in this screenshot,

2. Delete the following css code from the style.css file (line 1150 to 1170)

#mc-embedded-subscribe-form input[type="submit"],
.ctct-embed-signup .ctct-button {
background-color: #00aded;
border: 0;
color: #fff;
display: inline-block;
font-size: 1.071rem;
font-weight: 400;
text-transform: uppercase;
text-decoration: none;
cursor: pointer;
margin-bottom: 20px;
line-height: normal;
letter-spacing: 2px;
border-radius: 4px;
#mc-embedded-subscribe-form input[type="submit"]:hover,
.ctct-embed-signup .ctct-button:hover {
color: #333;
background-color: #fff;

3. Delete the following code from the /inc/options.php file (line 411 and 412)

#mc-embedded-subscribe-form input[type="submit"],

Let me know if that works.