Fixed background color in some node editor windows to fit with a dark theme

sdk/jme3-materialeditor/src/com/jme3/gde/materialdefinition/dialog/AddNodeDialog.form

@@ -8,6 +8,7 @@
   </Properties>
   <SyntheticProperties>
     <SyntheticProperty name="formSizePolicy" type="int" value="1"/>
+    <SyntheticProperty name="generateCenter" type="boolean" value="false"/>
   </SyntheticProperties>
   <AuxValues>
     <AuxValue name="FormSettings_autoResourcing" type="java.lang.Integer" value="0"/>

sdk/jme3-materialeditor/src/com/jme3/gde/materialdefinition/dialog/AddNodeDialog.java

@@ -204,7 +204,7 @@ public class AddNodeDialog extends javax.swing.JDialog {
     private void createDoc(ShaderNodeDefinition def) {
         JTextPane doc = new JTextPane();
         doc.setEditable(false);
-        doc.setBackground(new java.awt.Color(240, 240, 240));
+        //doc.setBackground(new java.awt.Color(240, 240, 240));
         doc.setMaximumSize(new java.awt.Dimension(300, 300));
         doc.setMinimumSize(new java.awt.Dimension(300, 300));
         doc.setPreferredSize(new java.awt.Dimension(300, 300));