DevTools: allow injecting stylesheets as themes.

Source/devtools/front_end/Settings.js

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 254 matching lines @@
     // Add currently running experiments here.
     this.fileSystemInspection = this._createExperiment("fileSystemInspection", "FileSystem inspection");
     this.canvasInspection = this._createExperiment("canvasInspection ", "Canvas inspection");
     this.cssRegions = this._createExperiment("cssRegions", "CSS Regions Support");
     this.frameworksDebuggingSupport = this._createExperiment("frameworksDebuggingSupport", "Enable frameworks debugging support");
     this.layersPanel = this._createExperiment("layersPanel", "Show Layers panel");
     this.stepIntoSelection = this._createExperiment("stepIntoSelection", "Show step-in candidates while debugging.");
     this.openConsoleWithCtrlTilde = this._createExperiment("openConsoleWithCtrlTilde", "Open console with Ctrl/Cmd+Tilde, not Esc");
     this.showEditorInDrawer = this._createExperiment("showEditorInDrawer", "Show editor in drawer");
     this.gpuTimeline = this._createExperiment("gpuTimeline", "Show GPU utilization on timeline");
+    this.applyCustomStylesheet = this._createExperiment("applyCustomStylesheet", "Allow custom UI themes");
 
     this._cleanUpSetting();
 }
 
 WebInspector.ExperimentsSettings.prototype = {
     /**
      * @return {Array.<WebInspector.Experiment>}
      */
     get experiments()
     {
@@ skipping 186 matching lines @@
     {
         // If there are too many breakpoints in a storage, it is likely due to a recent bug that caused
         // periodical breakpoints duplication leading to inspector slowness.
         if (breakpointsSetting.get().length > maxBreakpointsCount)
             breakpointsSetting.set([]);
     }
 }
 
 WebInspector.settings = new WebInspector.Settings();
 WebInspector.experimentsSettings = new WebInspector.ExperimentsSettings();