Make ui classes shared if applicable.

core/src/main/java/com/github/weisj/darklaf/ui/label/DarkLabelUI.java

@@ -44,7 +44,7 @@ import com.github.weisj.darklaf.util.PropertyKey;
  */
 public class DarkLabelUI extends BasicLabelUI implements PropertyChangeListener {
 
-    protected static final DarkLabelUI darkLabelUI = new DarkLabelUI();
+    protected static DarkLabelUI darkLabelUI;
 
     private Color inactiveForeground;
 
@@ -56,6 +56,7 @@ public class DarkLabelUI extends BasicLabelUI implements PropertyChangeListener
     }
 
     public static ComponentUI createUI(final JComponent c) {
+        if (darkLabelUI == null) darkLabelUI = new DarkLabelUI();
         return darkLabelUI;
     }
 

core/src/main/java/com/github/weisj/darklaf/ui/panel/DarkPanelUI.java

@@ -26,6 +26,7 @@ package com.github.weisj.darklaf.ui.panel;
 
 import javax.swing.*;
 import javax.swing.plaf.ComponentUI;
+import javax.swing.plaf.PanelUI;
 import javax.swing.plaf.basic.BasicPanelUI;
 
 /**
@@ -33,7 +34,10 @@ import javax.swing.plaf.basic.BasicPanelUI;
  */
 public class DarkPanelUI extends BasicPanelUI {
 
+    private static PanelUI darkPanelUI;
+
     public static ComponentUI createUI(final JComponent c) {
-        return new DarkPanelUI();
+        if (darkPanelUI == null) darkPanelUI = new DarkPanelUI();
+        return darkPanelUI;
     }
 }