Find style by it's unique identifier

        :Returns:
          Returns found style of type :class:`ooxml.doc.Style`.

def get_by_id(self, style_id, style_type = None):
        """Find style by it's unique identifier

        :Returns:
          Returns found style of type :class:`ooxml.doc.Style`.
        """

        for st in self.styles.values():
            if st:
                if st.style_id == style_id:
                    return st

        if style_type:
            return self.styles.get(self.default_styles[style_type], None)
        return None