As a Page Manager:
  1. Navigate to onMessage.
  2. Selecte Website > your site.
  3. To the right of the page you want to change the background color for, click the grey wrench icon.
  4. Select Change Master Layout (this is just to see what Layout is currently being used on this page).
  5. The Layout with the green check box is the layout being used.
  6. Click Cancel.
  7. On the top horizontal menu (below your Site Name), click Master Layouts.
  8. Select the Layout being used on your page. 
  9. In the upper left under the Layout name, click the Styles (paintbrush) icon.
  10. On the lower left of the page, click the Edit Styles button.
  11. Select Body.
  12. Click on the page to select the Body, and then in the upper left under Applied Style, you'll see the style being used.
  13. In the upper left corner above the layout name, click < View All Master Layouts.
  14. On the top horizontal menu (below your Site Name), click Styles.
  15. Search for your Style, and click Edit.
  16. Click on the Body in the style to select it.
  17. Scroll down on the left to the Background Color field.
  18. Enter/select your Background Color.
  19. Click Save in the lower right corner to save the changes.