You will have to make color changes to your colors of choice in the following codes: You add this to the WPS PRO custom CSS setting page.

For all WPS PRO buttons globally

.wps_button {
background-color: #E6E6E6;
background-image: linear-gradient(to bottom, #FFFFFF 0px, #E0E0E0 100%);
background-repeat: repeat-x;
border: 1px solid #D2D2D2;
border-radius: 3px;
box-shadow: 0 1px 0 rgba(255, 255, 255, 0.15) inset, 0 1px 1px rgba(0, 0, 0, 0.075);
color: #7C7C7C;
font-size: 13px;
font-weight: normal;
line-height: 19.5px;
padding: 3px 10px 3px 10px;
}

For Hover

.wps_button:hover {
color: #000;
}