How to use the @uifabric/utilities/lib.css function in @uifabric/utilities

let buttonsLocationCss = "";
    switch (buttonsLocation) {
      case CarouselButtonsLocation.top:
        buttonsLocationCss = styles.blockButtonsContainer;
        break;
      case CarouselButtonsLocation.center:
        buttonsLocationCss = styles.centralButtonsContainer;
        break;
      case CarouselButtonsLocation.bottom:
        buttonsLocationCss = styles.bottomButtonsContainer;
        break;
      default:
        return "";
    }

    const result = css(buttonDisplayModeCss, buttonsLocationCss);
    return result;
  }

private getButtonStyles(nextButton: boolean) {
    const buttonsDisplayMode = this.props.buttonsDisplay ? this.props.buttonsDisplay : CarouselButtonsDisplay.block;
    let result = "";
    if (buttonsDisplayMode === CarouselButtonsDisplay.buttonsOnly) {
      result = nextButton ? styles.buttonsOnlyNextButton : styles.buttonsOnlyPrevButton;
    }

    return css(result);
  }

private getMergedStyles = (componentStyles: string, userStyles?: ICssInput): string => {
    const mergedStyles = userStyles ? css(componentStyles, userStyles) : css(componentStyles);
    return mergedStyles;
  }