DevTools: allow injecting stylesheets as themes.

Source/devtools/front_end/Settings.js

Index: Source/devtools/front_end/Settings.js
diff --git a/Source/devtools/front_end/Settings.js b/Source/devtools/front_end/Settings.js
index 9d984db8195b6f22a78204728c5a6a27685074d3..8a4a808adf4915a9019cc3c630ef7c2f9e890acd 100644
--- a/Source/devtools/front_end/Settings.js
+++ b/Source/devtools/front_end/Settings.js
@@ -272,6 +272,7 @@ WebInspector.ExperimentsSettings = function()
     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();
 }