You can customize as per official documentation. To find exact class you need to override, do inspect and identify the class.
<ng-select class="custom"></ng-select>
.ng-select.custom {
border:0px;
min-height: 0px;
border-radius: 0;
}
.ng-select.custom .ng-select-container {
min-height: 0px;
border-radius: 0;
}
If you are using ViewEncapsulation, your should use special ::ng-deep selector which will prevent scoping for nested selectors.
.ng-select.custom ::ng-deep .ng-select-container {
min-height: 0px;
border-radius: 0;
}
https://www.npmjs.com/package/@ng-select/ng-select#custom-styles has more details around the same.