.modal-backdrop-custom {
    position: fixed;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100vh;
    background-color: #000;
    opacity: 0;
    transition: opacity 0.15s linear;
    z-index: 1040;
}

    .modal-backdrop-custom.show {
        opacity: 0.5;
    }