How to use the @covalent/echarts/base.getThemes function in @covalent/echarts

import { Injectable } from '@angular/core';
import { Observable, BehaviorSubject } from 'rxjs';

import { getThemes } from '@covalent/echarts/base';

@Injectable({ providedIn: 'root' })
export class ChartThemeSelectorService {
  selected: string = getThemes()[0];

  selectedSubject: BehaviorSubject = new BehaviorSubject(this.selected);

  selected$: Observable = this.selectedSubject.asObservable();

  select(theme: string): void {
    this.selected = theme;
    this.selectedSubject.next(theme);
  }
}