How to use the @uifabric/variants.getNeutralVariant function in @uifabric/variants

purpleLight: '#b4a0ff',
  blueDark: '#002050',
  blueMid: '#00188f',
  blue: '#0078d4',
  blueLight: '#00bcf2',
  tealDark: '#004b50',
  teal: '#008272',
  tealLight: '#00b294',
  greenDark: '#004b1c',
  green: '#107c10',
  greenLight: '#bad80a'
};

export const defaultTheme: ITheme = getTheme(true);

export const neutralTheme = getNeutralVariant(defaultTheme);
export const softTheme = getSoftVariant(defaultTheme);
export const strongTheme = getStrongVariant(defaultTheme);

export const invertedDefaultTheme: ITheme = createTheme({
  palette: invertedDefaultPalette,
  semanticColors: {
    bodyText: defaultTheme.palette.white,
    bodyBackground: defaultTheme.palette.neutralPrimary
  }
});

export const invertedPrimaryTheme: ITheme = createTheme({
  palette: invertedPrimaryPalette,
  semanticColors: {
    bodyText: defaultTheme.palette.white,
    bodyBackground: defaultTheme.palette.themePrimary