I am trying to make a simple map of the world in ArcMap 10.1, and I have a shapefile with all the countries in it. I have the symbology set to unique values, so the countries are all different colors. I want to add a gradient fill, but I can't add the gradient to its symbology without going through every single country and editing it. If I select Properties for All Symbols, it will apply the same color to every unique value. Any ideas?
Have a look at this blog page. It simulates sun tinting with gradient fills and gives a step by step guide. This should solve your problem.