Please add following code in Front End PM PRO > Settings > General > Custom CSS
.fep-button {
background-color: #ffffff !important;
}
.fep-button-active {
background-color: #b5b5b5 !important;
}
.fep-success {
background-color: #b5b5b5 !important;
}
#fep-header, #fep-content, #fep-footer {
border: 1px solid #333333;
}