Swing Nimbus L&F - Customized JInternalFrame title bar

I solved it! and it's not too hard either, so to be nice to people searching for the same thing, this is a solution I found:

I looked at this site: http://www.jasperpotts.com/blog/2008/08/skinning-a-slider-with-nimbus/, which details how to customize a nimbus slider, then, after looking at Java's documents about Nimbus values, I found one called

InternalFrame[Enabled+WindowFocused].backgroundPainter

Which is the painter which draws the title bar, and window border + background, so I took the code from the slider article, and modified it slightly, and this is what I got:

UIManager.put("InternalFrame[Enabled+WindowFocused].backgroundPainter", new Painter<Object>() {
			@Override
			public void paint(Graphics2D g, Object c, int w, int h) {
				g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
                g.setStroke(new BasicStroke(2f));
                g.setColor(Color.GRAY);
                g.fillRoundRect(0, 0, w-1, h-1, 8, 8);
                g.setColor(Color.WHITE);
                g.drawRoundRect(0, 0, w-1, h-1, 8, 8);
			}});
		
		UIManager.put("InternalFrame[Enabled].backgroundPainter", new Painter<Object>() {

			@Override
			public void paint(Graphics2D g, Object c, int w, int h) {
				g.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
                g.setStroke(new BasicStroke(2f));
                g.setColor(Color.RED);
                g.fillRoundRect(0, 0, w-1, h-1, 8, 8);
                g.setColor(Color.WHITE);
                g.drawRoundRect(0, 0, w-1, h-1, 8, 8);
			}});

All you need to do is paste this somewhere in your code, and you will get this result:

http://imgur.com/XuDGiHB
This is obviously very simple, but if you spend more time with it, you can make some amazing looking windows!