While I added support for page background colors in the RTF import back in 2013, the export part was missing up to now.
If you set a solid color fill for a page style, and you export it to RTF, here is how the reference rendering output looks like:
However, in Libreoffice only the background of the paragraph reflected the color set by the user:
After implementing this feature in the RTF export filter, it now looks much closer to the reference:
At the moment only solid fill is implemented, so other advanced fill types like graphics or gradients are still missing.