German 'Marke Individuell' Stamps As Thematics?

Hi

Just wondered how the user generated but valid postal stamps are viewed as thematics. For example, the German 'Marke Individuell' series.

For one of my thematics interests, aspects of the history of maths, there's a series of 21 mathematicians with a uniform design brief over the set.

But they seem to be pricey either as singles or as the set and haven't acquired the series. EDIT. Example below of John Napier who invented logarithms from the set.

Is there any way of finding out who instigated a set in this German series, this set looks professionally designed, perhaps by a German maths society or institution.

Thanks for any insight.

regards
David