It will not work like that. What you can do is use :

<infobox theme-source="conditionaltheme">

Then simply use a template to call it like this:

 {{myinfobox|conditionaltheme={{{#if:{{{hidden|}}}|hidden|}}

Or simply call it like this:

<infobox theme-source="{{#if:{{{hidden|}}}|hidden|}}">

Uhm, no…

<infobox theme-source="{{#if:{{{Condition|}}}|hidden|}}">

…does not compute. See here

Hmm, that's odd. The first option I gave does work at least.

Or alternatively you can try this:

  <infobox theme="defaultTheme" theme-source="Versteck">

I had actually forgotten that wikitext is only allowed within certain tags, e.g. <format> <navigation>, etc...

Edit: As long as {{{versteck}}} contains the word 'hidden' it'll change the theme to 'pi-theme-hidden' .

I think this is a missing feature. I need to set the theme based on more complex conditions.

Not really. Keeping things simple is more important than accomodating all edge cases. It is easy enough to have a helper template that will  process all those complex conditions and return the needed theme. 

 /*ChooseTheme*/

{{{#if:{{{hidden|}}}|hidden|}}

 {{myinfobox|conditionaltheme={{choosetheme}}

Allowing wikitext in attributes such as theme-source may make these infobox templates unnecessarily complex especially when they deploy the infoboxbuilder that will have to interpret these attributes. Some people might start adding dozens of lines of conditional statements, which is not desirable.

btw: I solved this by creating two separate templates

Great.