Pr. Karza's games are often criticised for pasted-on themes, despite very ingenious mechanics. Is this the case here or not?

Yes.  This is not your typical FFG game, but don't let that keep you from trying it.  The hand management and plan ahead aspects of the game are terrific, and the "risks" add an element of fun and excitement that keep this from being a typical "auction" game.  Definitely recommended, especially if it's still available in the FFG "Holiday Sale".

TK

 Actually I rather think the mechanics are pasted onto the them, not the other way round.

The game has a mechanic (all games do) and in this game the mechanic is tweeked and pulled at every episode to match the plot of the poem.

Rather than Knizia saying "I have a mechanic, do you have a them?" I think the producers said "We want to make a game about Beowulf, do you have a mechanic that can be adapted to fit?" and Knizia said "I'll have a look in my library of mechanics and see what would serve best"

I think, in this case the theme has nothing to do with the mechanic. It could have been any other theme as well (or no theme at all). Compared to Knizias great Lord-of-the-Rings adaptation, Beowulf was fairly dissappointing.

Could someone tell me what this original mechanic is?  Does it work like the story-adaptation in Arabian Nights or something?

Pulsar said:

It's more about resource management and risk. There are a series of challenges, and you can't win them all. To win the game you have to know when to play your cards to win a challenge, and when to hold them back and lose.

... like that Kenny Rogers song :)