Creates a theme with gradient colors.
The gradient is created given the colors and limit values of the
chosen property.
The property must have numeric values.
If the min is set, all smaller values will be displayed with the first
gradient color.
If the max is set, all larger values will be displayed with the last
gradient color.
Creates a theme with colors by rule.
The feature color is obtained from the first rule that returns
a non-null color.
If all rules return a null color, the default color is used.