It is possible to modify the appearance of buttons utilized for certain actions such as "Start Customization" and "Share".
Hence, an option can be located under General > Display that allows you to modify the CSS class for said buttons. This feature proves to be beneficial if you desire to apply the identical style to these buttons as the one used for the theme buttons.
In case you wish to apply your own CSS style, feel free to utilize this CSS class for making style modifications. For instance:
.fpd-secondary-btn {
background: #f9f9f9;
font-size: 20px;
font-weight: bold;
}