This example shows how to use a FeatureTypeClassifier to modify an existing rule group of an SLD.
Expand the "Population" group in the grid to see the current classification. Then press the "Reclassify" button in the toolbar to create a new classification of 4 classes with natural breaks and a color ramp from the first and third colors of the original classification.
The js is not minified so it is readable. See featuretype-classifier.js.