Within the stylesheet Notebook you need to change the Option:
Applying this is complicated by the default stylesheet settings that disable evaluation.
Here is Palette generating code to create a
Button to apply it:
SetOptions[SelectedNotebook, StyleDefinitions -> "StylesheetFormatting.nb"]]
Evaluating that code should create a palette that looks like this:
Open and focus the private style sheet, then click that button and it should immediately apply the change.
Otherwise use a text editor on the Notebook and replace