Why glyphicon is not showing?

import { Component, OnInit, Input } from '@angular/core';

@Component({
  selector: 'zippy',
  templateUrl: './zippy.component.html',
  styleUrls: ['./zippy.component.css']
})
export class ZippyComponent  {
  @Input('title') title: string;
  isExpanded: boolean;

  toggle() { 
    this.isExpanded = !this.isExpanded;
  }

}

<div class="zippy">
  <div 
    class="zippy-heading"
    [class.expanded]="isExpanded"
    (click)="toggle()"
    >
    {{ title }}
    <span class="glyphicon"
      [ngClass]="{
        'glyphicon-chevron-up': isExpanded,
        'glyphicon-chevron-down': !isExpanded
      }"
    ></span>
  </div>
  <div *ngIf="isExpanded" class="zippy-body">
    <ng-content></ng-content>
  </div>
</div>

styleUrls:['./zippy.component.css','./app/styles.css']

Failed to compile.

./src/app/zippy/zippy.component.ts
Module not found: Error: Can't resolve './app/styles.css' in 'C:\Demo\Angular\tweet\src\app\zippy'
 @ ./src/app/zippy/zippy.component.ts 27:51-78
 @ ./src/app/app.module.ts
 @ ./src/main.ts
 @ multi webpack-dev-server/client?http://localhost:4200 ./src/main.ts