Themes are the mechanism by which you determine the map display. This can be as simple as a single style for all features (override) or more complex rules to visualize the information from the underlying data.

The main purpose of a theme is to graphically communicate a particular type of data that is associated with the layer, such as population levels or temperature ranges. The particular type of data forms the theme of the map, and the theme can represent the data using shades of color, fill patterns, symbols, bar charts, and pie charts.

For example, a ranged theme shows color blocks where each color represents features on the map that meet the same criteria. A graduated symbol theme is useful for showing distributions of populations for example, with the largest symbol representing the largest population.

Themes can also be created for labels. For example, use a ranged label theme to show the relative population size among cities. The largest labels represent the cities with the largest populations.