.is-invalid, .form-control.is-invalid {
    border-color: #dc3545 !important; /* Red border */
    padding-right: calc(1.5em + 0.75rem); /* Adjust padding for feedback icon */
    background-image: url("data:image/svg+xml,..."); /* Feedback icon */
    background-repeat: no-repeat;
    background-position: right calc(0.375em + 0.1875rem) center;
    background-size: calc(0.75em + 0.375rem) calc(0.75em + 0.375rem);
}

.is-invalid:focus {
    border-color: #dc3545; /* Focus border color */
    box-shadow: 0 0 0 0.25rem rgba(220, 53, 69, 0.25); /* Red shadow */
}

.invalid-feedback {
    display: block !important;
    width: 100%;
    margin-top: 0.25rem;
    margin-bottom: 0.25rem;
    font-size: 80%;
    color: #dc3545;
}