When creating a book layout in Writer using Page Styles, sometimes it looks quite nice when you have a solid colour or graphic in the background, and a white border around the edge of the page. This exports quite nicely to PDF.
To achieve this look, simply go Format >> Page and edit the page margins to achieve the desired thickness.
You can even only adjust one or two sides to get some interesting effects.
To change the background colour, click on the background link at the top of the Page Setup window, and select the desired colour.