To change the style of your CF7 forms you would need to edit the CSS style sheets used by your WordPress theme.
See Styling Contact Form for a general explanation of styling CF7 forms using CSS.
There is a link at the bottom of the page to a comprehensive and detailed article on Styling Contact Form 7 Forms. The article shows people, with suitable HTML & CSS skills, how to change the appearance of their Contact Form 7 Forms to meet their particular requirements.
You can target the CF7 form button with the code below
.wpcf7-form input[type="submit"] {
/* add a suitable width/height etc*/
width: 70px;
height: 30px;
font-size: 13px;
color: #ccc1bc;
/* change cursor to hand*/
cursor: pointer;
}
Add your CSS via Child Theme or use custom CSS plugin.