If the group headers ("today", "yesterday" etc) lack contrast and are difficult to read, the only option you have is to shade the group heading. Shading makes the group header smaller, shaded and uses black text.
Click on the row of field names in the message list and choose Custom…, Other Settings.
This setting is one of the very few global settings used in Views and as such, it applies to all views and resetting a view won't change this option. You can enable it in any view and disable it in any view and the change applies to all views.
Note that you may need to refresh a view (change to another folder and come back) for the shading to change in some views.
Published September 14, 2007. Last updated on November 4, 2013.