How to use the perfect-scrollbar/css/perfect-scrollbar.css.toString function in perfect-scrollbar

import PerfectScrollbarType, * as PerfectScrollbarNamespace from 'perfect-scrollbar';
import psStyles from 'perfect-scrollbar/css/perfect-scrollbar.css';

import { OptionsContext } from '../components/OptionsProvider';
import styled, { createGlobalStyle } from '../styled-components';

/*
 * perfect scrollbar umd bundle uses exports assignment while module uses default export
 * so when bundled with webpack default export works but with jest it crashes
 * That's why the following ugly fix is required
 */
const PerfectScrollbarConstructor =
  PerfectScrollbarNamespace.default || ((PerfectScrollbarNamespace as any) as PerfectScrollbarType);

const PSStyling = createGlobalStyle`${psStyles && psStyles.toString()}`;

const StyledScrollWrapper = styled.div`
  position: relative;
`;

export interface PerfectScrollbarProps {
  options?: PerfectScrollbarType.Options;
  className?: string;
  updateFn?: (fn) => void;
}

export class PerfectScrollbar extends React.Component {
  private _container: HTMLElement;
  private inst: PerfectScrollbarType;

  componentDidMount() {