Nimbus appears to work differently than other LAF's. You need to set the property before you set the LAF:
UIManager.put("ToolTip.font", new Font("SansSerif",Font.PLAIN,25));
try
{
for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels())
{
if ("Nimbus".equals(info.getName()))
{
UIManager.setLookAndFeel(info.getClassName());
break;
}
}
}
catch (Exception e)
{
// If Nimbus is not available, you can set the GUI to another look and feel.
}
With other LAFs I believe you can change any property before you create the first component of that type.