flow-based How do I change the background color for JPanels using Nimbus Look and Feel? you can change and assign one value to something from Nimbus Defaults ,
but you are sure that you need this output for the GUI, nothing nice

vs basic JButton with Nimbus L & F

from code
import java.awt.*; import javax.swing.*; import javax.swing.plaf.InsetsUIResource; public class NimbusJPanelBackGround { public NimbusJPanelBackGround() { JButton btn = new JButton(" Whatever "); JButton btn1 = new JButton(" Whatever "); JPanel p = new JPanel(); p.add(btn); p.add(btn1); JFrame f = new JFrame(); f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); f.add(p, BorderLayout.CENTER); f.setSize(200, 100); f.setLocation(150, 150); f.setVisible(true); } public static void main(String[] args) { try { for (UIManager.LookAndFeelInfo laf : UIManager.getInstalledLookAndFeels()) { if ("Nimbus".equals(laf.getName())) { UIManager.setLookAndFeel(laf.getClassName()); UIManager.getLookAndFeelDefaults().put("Panel.background", Color.white); UIManager.getLookAndFeelDefaults().put("Button.contentMargins", new InsetsUIResource(0,0,0,0)); } } } catch (Exception e) { e.printStackTrace(); } EventQueue.invokeLater(new Runnable() { @Override public void run() { NimbusJPanelBackGround nimbusJPanelBackGround = new NimbusJPanelBackGround(); } }); } }
earlier +1 for an interesting question
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