DevTools ext docs: Add chrome.devtools.panels.themeName

chrome/common/extensions/api/devtools/panels.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "devtools.panels",
     "description": "Use the <code>chrome.devtools.panels</code> API to integrate your extension into Developer Tools window UI: create your own panels, access existing panels, and add sidebars.",
     "nocompile": true,
     "types": [
@@ skipping 280 matching lines @@
       }
     ],
     "properties": {
       "elements": {
         "$ref": "ElementsPanel",
         "description": "Elements panel."
       },
       "sources": {
         "$ref": "SourcesPanel",
         "description": "Sources panel."
+      },
+      "themeName": {
+        "type": "string",
+        "description": "The name of the color theme set in user's DevTools settings. Possible values: <code>default</code> (the default) and <code>dark</code>."
       }
     },
     "functions": [
       {
         "name": "create",
         "type": "function",
         "description": "Creates an extension panel.",
         "parameters": [
           {
             "name": "title",
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             "name": "callback",
             "type": "function",
             "optional": true,
             "description": "A function that is called when the resource has been successfully loaded."
           }
         ]
       }
     ]
   }
 ]