Custom Themes allow a user to create a theme in the legend that suits the users needs. Making a custom theme can allow a user to see data in a different way to present to a customer, or just to see the data in a different way.
Themes are a resource, much like many other resources that are Domain Specific, but not year-specific. Meaning they can be setup for a certain grower, but they will persist year after year until they are removed.
These are used when you have Nominal or Group type attributes. Since they aren't numbers that fall in a range, you can specify colors for each value.
These Themes are used on the map to define the areas on the map when their domain is selected on the resource tree
There are 4 default themes, to use them you need to use the defined theme name. DefaultGrower, DefaultFarm, DefaultField, and DefaultCropZone.