This is similar to bug 42779. The method that pains the shadow around a Writer page is SwPageFrm::PaintBorderAndShadow(): http://opengrok.libreoffice.org/xref/core/sw/source/core/layout/paintfrm.cxx#5669 Right now, the code actually uses the entire image as an Alpha only; it would be cleaner to just take the image, and apply the Alpha from the image. It would be easier for designers to do changes after this is fixed. More pointers - the image that is parsed there is under icon-themes/galaxy/sw/res/page-shadow-mask.png.
adding LibreOffice developer list as CC to unresolved Writer EasyHacks for better visibility. see e.g. http://nabble.documentfoundation.org/minutes-of-ESC-call-td4076214.html for details
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