当mat-menu打开时，修改mat-按钮背景颜色

<button mat-button [matMenuTriggerFor]="menu" [class.menu-open]="isMenuOpen">Open Menu</button>
<mat-menu #menu="matMenu">
  <button mat-menu-item>Item 1</button>
  <button mat-menu-item>Item 2</button>
  <button mat-menu-item>Item 3</button>
</mat-menu>

.menu-open {
  background-color: #ff0000; /* 设置背景颜色为红色 */
}

import { Component } from '@angular/core';

@Component({
  selector: 'app-menu-example',
  templateUrl: './menu-example.component.html',
  styleUrls: ['./menu-example.component.css']
})
export class MenuExampleComponent {
  isMenuOpen = false;

  toggleMenu() {
    this.isMenuOpen = !this.isMenuOpen;
  }
}

<button mat-button [matMenuTriggerFor]="menu" [class.menu-open]="isMenuOpen" (click)="toggleMenu()">Open Menu</button>