// you can override css of any node module or library by :host and ::ng-deep.

:host ::ng-deep .multiselect-dropdown .dropdown-btn {
    display: inline-block;
    border: 1px solid #adadad;
    width: 100%;
    padding: 6px 12px;
    margin-bottom: 0;
    font-size: 12px;
    font-weight: 400;
    line-height: 1.1;
    text-align: left;
    vertical-align: middle;
    cursor: pointer;
    background-image: none;
    border-radius: 4px;
}