CSS

ng-multiselect-dropdown custom css

//Angular by default adds some _ngcontent-xx to your component CSS file
//so that it won't conflict with other components.
//To solve your problem you need to add below CSS in your global style.css file or
//another way to make your component as encapsulation: ViewEncapsulation.None meaning its CSS
//won't append default classes of Angular.

//Solution 1: Add in global stylesheet.
style.css
.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;
}

//Solution 2 Make component ViewEncapsulation.None
component.ts
import { Component } from '@angular/core';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ],
  encapsulation: ViewEncapsulation.None // Add this line
})
export class AppComponent  {

}

ng-multiselect-dropdown custom css

// 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;
}