The problem only occurs when the size "large" is selected at a shipped theme (any other size works; after changing a size -> select "rebuild with all images" and the white borders are gone again). And the custom themes dont work either.
I can't figure out why, but at least it shows me that it is not my fault.
Others have the same problem, too:
So a workaround is to use a shipped theme at the extra-large setting. My printing provider scales the pdf accordingly, but I couldnt figure out how they handle the different paper-aspect-ratios.
This behaviour is only happening, when a printer (in my case a virtual printer) is installed. If no printer at all is installed, there is no way (I could find) to get no borders in any theme and size.
Here you can find a way to install a virtual printer:
http://support.quark.com/Knowledgebase/ArticleDetails.aspx?kbarticleid=bc63f28b- 1c94-de11-93a0-00155d642a02&l=1033&sj=9a11259a-b9a7-4069-98ff-a91d815951b4&pi=0& ps=10&st=0&sb=&so=&kd=&ui=en-US&title=How%20to%20Create%20a%20Virtual%20Printer
Using Cups-PDF to have a "real" printer (program that prints to pdf) has the same effect as the virtual printer.