'Add a new Format to the Excel Workbook.
Args:
properties: The format properties.
Returns:
Reference to a Format object.'

def add_format(self, properties=None):

    format_properties = self.default_format_properties.copy()
    if self.excel2003_style:
        format_properties = {'font_name': 'Arial', 'font_size': 10, 'theme': (1 * (-1))}
    if properties:
        format_properties.update(properties)
    xf_format = Format(format_properties, self.xf_format_indices, self.dxf_format_indices)
    self.formats.append(xf_format)
    return xf_format
