setElementStyle() method is used to modify the display style of a static form widget. The accepts two arguments: the first one is the name of a static form widget and the second one is the name of a class in a theme file (.qxtheme):
CALL fo.setElementStyle("fonronly.main_label", "my_style_class")
A class in a theme file is created using the With Class filter in Lycia Theme Designer. The properties set for the specified class will then be applied to the selected widget, even if this class was not applied to this widget directly in the theme file. The function of this method is the same as the function of the Apply Class filter of Lycia Theme Designer used for the same widget, but the method can apply classes dynamically.
setFieldStyle() method has a synonym - setElementStyle() - that is used for compatibility purpose.