What do you want to achieve exactly? If you just want to modify the appearance of LibreOffice, without modifying the output, you could set a custom color for the document background: Menu "
Tools" -> "
Options" -> "
Appearance" -> "
If you want to modify the output, too, you will need to use a workaround, since you can only modify the area inside the page margins (as laurent described).
To modify the page appearance in a printable way, there's a nice workaround: Don't touch the page margins, but insert the content (full-page watermark, stationery, or just a full-page frame with a certain background) into the header / footer. This way, it will modify each new page, too. To modify the "background color" of the complete page, you could proceed as follows:
- Insert a header or footer, and put the cursor there;
- Select Menu "
Insert" -> "
- Modify the frame properties:
- on "
Type" tab: frame size to paper size, bound to paragraph, Frame Position: "
- on "
Wrap" tab: "
Through"; Wrap Options: "
- no borders,
- background color as desired,
That's it - now you have a frame as "background object" on every page, thus effectively setting the page background.
See also the accepted answer in this ask.libreoffice.org thread.