The map theme is the sum of all the style sets defined
in the map layers.
In addition to data sources, most layers also contain
styling information, for example,
about grids, labels, terrain analysis, measurements. See
Themes and styling for
more details about styling in maps.
Most map layers are used to manage graphic objects created
from map features imported from various map data files. The styling
options differ according to the content of each file and the map features
imported by the different readers.