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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def css(self, mapping=None):
        '''Update the css dictionary if ``mapping`` is a dictionary, otherwise
        return the css value at ``mapping``.

        If ``mapping`` is not given, return the whole ``css`` dictionary
        if available.
        '''
        css = self._css
        if mapping is None:
            return css
        elif isinstance(mapping, Mapping):
            if css is None:
                self._extra['css'] = css = {}
            css.update(mapping)
            return self
        else:
            return css.get(mapping) if css else None