don't use any longer the page background color white as being the default (equal...

don't use any longer the page background color white as being the default (equal no color) because document classes might have other colors as default
(The default color is still internally white because we have to set a color but the color is only used when the user explicitly specified it.)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34058 a592a061-630c-0410-9148-cb99ea01b6c8