How to use the @uifabric/fluent-theme.FontSizes.size12 function in @uifabric/fluent-theme

}
    ],
    itemName: {
      fontSize: '15px',
      whiteSpace: 'nowrap',
      overflow: 'hidden',
      textOverflow: 'ellipsis'
    },
    itemLabel: [
      {
        fontSize: FontSizes.size12,
        fontWeight: FontWeights.bold
      },
      globalClassNames.listElement
    ],
    timeStamp: [{ fontSize: FontSizes.size12 }, globalClassNames.timeStamp],
    subComponentStyles: {
      pivot: pivotStyles
    }
  };
};

borderBottom: `1px solid ${theme.semanticColors.bodyDivider}`,
        display: 'flex',
        selectors: {
          '&:hover': { background: theme.palette.neutralLight }
        }
      }
    ],
    itemName: {
      fontSize: '15px',
      whiteSpace: 'nowrap',
      overflow: 'hidden',
      textOverflow: 'ellipsis'
    },
    itemLabel: [
      {
        fontSize: FontSizes.size12,
        fontWeight: FontWeights.bold
      },
      globalClassNames.listElement
    ],
    timeStamp: [{ fontSize: FontSizes.size12 }, globalClassNames.timeStamp],
    subComponentStyles: {
      pivot: pivotStyles
    }
  };
};