Implement basic theming for panel titlebars.

chrome/app/nibs/Panel.xib

 <?xml version="1.0" encoding="UTF-8"?>
 <archive type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="7.10">
         <data>
                 <int key="IBDocument.SystemTarget">1050</int>
                 <string key="IBDocument.SystemVersion">10K549</string>
                 <string key="IBDocument.InterfaceBuilderVersion">804</string>
                 <string key="IBDocument.AppKitVersion">1038.36</string>
                 <string key="IBDocument.HIToolboxVersion">461.00</string>
                 <object class="NSMutableDictionary" key="IBDocument.PluginVersions">
                         <string key="NS.key.0">com.apple.InterfaceBuilder.CocoaPlugin</string>
@@ skipping 27 matching lines @@
                         </object>
                         <object class="NSCustomObject" id="1004">
                                 <string key="NSClassName">NSApplication</string>
                         </object>
                         <object class="NSWindowTemplate" id="1005">
                                 <int key="NSWindowStyleMask">3</int>
                                 <int key="NSWindowBacking">2</int>
                                 <string key="NSWindowRect">{{196, 240}, {480, 270}}</string>
                                 <int key="NSWTFlags">544736256</int>
                                 <string key="NSWindowTitle">Window</string>
-                                <string key="NSWindowClass">ChromeEventProcessingWindow</string>
+                                <string key="NSWindowClass">ThemedBrowserWindow</string>
                                 <nil key="NSViewClass"/>
                                 <string key="NSWindowContentMaxSize">{1.79769e+308, 1.79769e+308}</string>
                                 <object class="NSView" key="NSWindowView" id="1006">
                                         <reference key="NSNextResponder"/>
                                         <int key="NSvFlags">256</int>
                                         <string key="NSFrameSize">{480, 270}</string>
                                         <reference key="NSSuperview"/>
                                 </object>
                                 <string key="NSScreenRect">{{0, 0}, {1680, 1028}}</string>
                                 <string key="NSMaxSize">{1.79769e+308, 1.79769e+308}</string>
@@ skipping 285 matching lines @@
                                         </object>
                                 </object>
                                 <object class="IBPartialClassDescription">
                                         <string key="className">PanelWindowControllerCocoa</string>
                                         <string key="superclassName">NSWindowController</string>
                                         <object class="IBClassDescriptionSource" key="sourceIdentifier">
                                                 <string key="majorKey">IBUserSource</string>
                                                 <string key="minorKey"/>
                                         </object>
                                 </object>
+                                <object class="IBPartialClassDescription">
+                                        <string key="className">ThemedBrowserWindow</string>
+                                        <string key="superclassName">ChromeEventProcessingWindow</string>
+                                        <object class="IBClassDescriptionSource" key="sourceIdentifier">
+                                                <string key="majorKey">IBUserSource</string>
+                                                <string key="minorKey"/>
+                                        </object>
+                                </object>
                         </object>
                         <object class="NSMutableArray" key="referencedPartialClassDescriptionsV3.2+">
                                 <bool key="EncodedWithXMLCoder">YES</bool>
                                 <object class="IBPartialClassDescription">
                                         <string key="className">NSApplication</string>
                                         <string key="superclassName">NSResponder</string>
                                         <object class="IBClassDescriptionSource" key="sourceIdentifier" id="513745449">
                                                 <string key="majorKey">IBFrameworkSource</string>
                                                 <string key="minorKey">AppKit.framework/Headers/NSApplication.h</string>
                                         </object>
@@ skipping 619 matching lines @@
                 </object>
                 <object class="NSMutableDictionary" key="IBDocument.PluginDeclaredDevelopmentDependencies">
                         <string key="NS.key.0">com.apple.InterfaceBuilder.CocoaPlugin.InterfaceBuilder3</string>
                         <integer value="3000" key="NS.object.0"/>
                 </object>
                 <bool key="IBDocument.PluginDeclaredDependenciesTrackSystemTargetVersion">YES</bool>
                 <string key="IBDocument.LastKnownRelativeProjectPath">../../chrome.xcodeproj</string>
                 <int key="IBDocument.defaultPropertyAccessControl">3</int>
         </data>
 </archive>