This option provides default output properties for widgets dragged from the Database tree to the Design canvas. By default, the option is set like this:

pui["default output properties"] = {
  "field type": "output field",
  "css class": "outputField",
  "css class 2": "blueprint-defaults",
  "css class 3": "blueprint-output-field"
};

The property names can be any valid property name shown in the Properties window. Property names are case-sensitive. Property values should be specified exactly as they would be in the Properties window. The values should always be specified as strings, even if the data represents numeric or boolean values.

For example, to change the default styling from Blueprint to Office Copy:

 pui["default output properties"] = {
  "field type": "output field",
  "css class": "outputField",
  "css class 2": "office-copy-defaults",
  "css class 3": "office-copy-output-field"
};

Where do you save this configuration option?  Here.