If you're talking about changing the color of the background for the entire program, go to Modify/File/Properties and click on the colored square in front of the word 'Background' toward the top of the box.
If you just want to change the background for a section, you drag a display icon (the top one on the panel) onto the flowline and double click on it, then grab the square box tool and draw a large box on the window--you can make it bigger than the presentation window by stretching it larger and moving it. To change the color, select Window/Inspectors/Colors (or control-K) and choose the color you want from the color picker.
steve