What version of Word?
In Word 2007, Page Layout tab, use the Page Color tool to set the background
to black. Ordinarily, font color is set to Automatic, so just setting the
Page Color to black gets you white text on a black background.
In Word 2003, choose Format - Background, and click on a black color. Again,
with font color set to Automatic, this gets you white on black.
I don't recommend printing such documents, though.