I tried doing
UIManager.getDefaults().put(\"TitledBorder.font\", Font.BOLD);
contentPanel.setBorder(new TitledBorder(\"Client Downloader\"));
You mark the question as accepted, but the comment says its not working. I would agree it should not be working.
Font.BOLD
is not a Font. It is a property of a Font. If you want to change the font you can do:
TitledBorder border = new TitledBorder(...);
border.setTitleFont( border.getTitleFont().deriveFont(Font.BOLD + Font.ITALIC) );
I added the italic just to show you the code works, since it appears to me that in the Metal LAF the default is for a Bold font.