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!