Hello Francesco

Thank you for your purchase.

I see that page already includes black, but to change the colors you can use the following CSS:

.amelia-app-booking #am-step-booking .am-select-date, .amelia-app-booking .am-step-booking-catalog .am-select-date {
background: linear-gradient(135deg, lightblue 0%, blue 100%);
}
button.el-button.el-button--primary {
background-color: blue !important;
}
button.el-button.el-button--primary:hover {
background-color: lightblue !important;
}