An gtTheme stores a theme to visualise vector and raster
objects. It is recommended to use setTheme to modify a
gtTheme, because it carries out all the checks and makes sure that
names of the parameters are properly matched.
title[named list(3)] properties of the title.
box[named list(4)] properties of the bounding box.
xAxis[named list(5)] properties of the x-axis, its labels
and ticks.
yAxis[named list(5)] properties of the y-axis, its labels
and ticks.
grid[named list(5)] properties of the major and minor grid.
legend[named list(10)] properties of the legend, its title,
labels, ticks and bounding box.
vector[named list(7)] properties of a vector object.
raster[named list(2)] properties of a raster object.