def cell_styles(self):
        """dict of {(row name, col name): style}"""
        styles = {}
        for colname, col in self.dataframe.items():
            for rowname, value in col.items():
                if isinstance(value, Value) and value.style is not None:
                    style = value.style
                    if not isinstance(style, CellStyle):
                        style = self._named_styles[style]
                    styles[(rowname, colname)] = style
        return styles
dict of {(row name, col name): style}