Have you looked into creating a custom look and feel using properties
you've defined?

IE

public class MyLookAndFeel extends MetalLookAndFeel
{
    protected void initComponentDefaults(UIDefaults table)
    {
        super.initComponentDefaults();
        Object [] uiDefaults =
        {
            "my.big.font", new Font ("Arial, Font.PLAIN, 20))
            //etc
        }
        table.putDefaults(uiDefaults);
    }
}

Then when the user clicks to change font:

UIManager.setLookAndFeel (new MyLookAndFeel());

The trick is that all of your JComponents then need to override
updateUI().  In updateUI() you can load the fonts using something
like:

myJLabel.setFont(UIManager.getFont("my.big.font"));

Btw, for anyone reading this, I would like to know if this approach is
considered a big no-no, or if it's pretty reasonable.  Obviously it's
a bit of a pain in the butt needing to override updateUI() all over
the place.