This issue is caused by the desired behaviour of nimbus, the workaround offered by Sun has no effect, so I'm going to brute force a fix, which whilst horrible works nicely across all platforms.

Fixes issue 3743: nimbus LAF ignores background colour for topic bar

Change-Id: Id6274a362572b7281b39e39763463cb78e433a06
Reviewed-on: http://gerrit.dmdirc.com/866
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>

src/com/dmdirc/addons/ui_swing/components/TextPaneInputField.java

@@ -30,7 +30,10 @@ import com.dmdirc.ui.interfaces.InputField;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 
+import javax.swing.BorderFactory;
 import javax.swing.JEditorPane;
+import javax.swing.plaf.ComponentUI;
+import javax.swing.plaf.basic.BasicEditorPaneUI;
 import javax.swing.text.BadLocationException;
 
 /**
@@ -94,4 +97,18 @@ public class TextPaneInputField extends JEditorPane implements InputField {
     public void removeActionListener(ActionListener listener) {
         //Ignore
     }
+
+    /** {@inheritDoc} */
+    @Override
+    protected void setUI(final ComponentUI newUI) {
+        super.setUI(new BasicEditorPaneUI());
+        super.setBorder(BorderFactory.createEmptyBorder(3, 3, 3, 3));
+    }
+
+    /** {@inheritDoc} */
+    @Override
+    public void updateUI() {
+        super.setUI(new BasicEditorPaneUI());
+        super.setBorder(BorderFactory.createEmptyBorder(3, 3, 3, 3));
+    }
 }

src/com/dmdirc/addons/ui_swing/components/TopicBar.java

@@ -43,8 +43,8 @@ import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import java.awt.event.MouseEvent;
 import java.awt.event.MouseListener;
-import javax.swing.AbstractAction;
 
+import javax.swing.AbstractAction;
 import javax.swing.JButton;
 import javax.swing.JComponent;
 import javax.swing.JLabel;