Example how to change Sign Up Form with Custom CSS

/* Change background color and text for hole web sign up form */
body {
background: #000!important;
color: #FFF;
}
/* Change size for your logo/image */
.brand-logo {
width: 256px;
}
/* Change color Form's Fields */
.form-control {
border: solid #E91E63;
}
/* Change button colors */
.btn-dark {
--bs-btn-color: #FFF;
--bs-btn-bg: #F44336;
--bs-btn-border-color: #212529;
}
/* Change text color for The disclaimer */
.data-rates {
color: #28a745;
}
/* Change frame for thank you message */
.alert-success {
--bs-alert-color: #FFF;
--bs-alert-bg: #F44336;
--bs-alert-border-color: #E91E63;
}