I just clicked though all the prizes for the various stages. Nothing turns up for 1.000 items. Does anybody know, what the reward is?

I think it unlocks the Autumn site theme

Ahhh... Thx! Still it should be mentioned, when pressing the star, like everything else :-(

all site themes are bugged like that; if an achievement has a star that indicates it grants a reward, but clicking the star doesn't show anything, that's how you know the reward is a site theme.