When page is set <100% how to color background?

you will need to edit /templates/tmpl_uni/css/general.css
I guess body block.

Regards
Artur

go to > templates > tmpl_uni > css > general.css

find

background-image:url('../images/body_backgr.gif');

that's the path to the background image, you'll have to edit that image..

or if you want only a simple color as background remove that line of code and add:

background-color: #FFFFFF;

that's a hex color, search in google for hex color picker to get the color you want and then replace "FFFFFF" - which means white to what you want..