As a Page Manager: 
  1. Navigate to onMessage.
  2. Select Website > Your Site.
  3. Click Master Layouts.
  4. Select the desired Master Layout.
  5. Select the Styles tab in the upper left corner. 
  6. At the bottom of the window, click Edit Styles and select Regions.
  7. Click the region and use the style drop down in the upper left to select the style.
  8. Click Apply Changes, when satisfied with the changes.