The top bar in Unity 2017.1.0a6 is black on Mac.

Steps to reproduce:
1. Create a new project

Reproduced with: 2017.1.0a6.
Not reproduced with: 5.6.0f3 and older.

Expected behavior: the Window bar is usual - standard Mac icons for Closing, Minimizing and Maximizing the window; a name of the app.
Actual behavior: the Window bar is all black

Note:
The buttons actually exist and work, however, they are not visible.
The issue appears with an Error in the Console:
InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEngine.Experimental.UIElements.UIElementsUtility.EndContainerGUI () (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:174)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:109)
UnityEngine.Experimental.UIElements.IMGUIContainer.HandleEvent (UnityEngine.Event evt, UnityEngine.Experimental.UIElements.VisualElement finalTarget) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:137)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoRepaint (IStylePainter args) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/IMGUIContainer.cs:35)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:374)
UnityEngine.Experimental.UIElements.Panel.PaintSubTree (UnityEngine.Event e, UnityEngine.Experimental.UIElements.VisualElement root, Matrix4x4 offset, Rect currentClip, Matrix4x4 clipTransform, Rect currentGlobalClip) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:382)
UnityEngine.Experimental.UIElements.Panel.Repaint (UnityEngine.Event e) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/Panel.cs:393)
UnityEngine.Experimental.UIElements.UIElementsUtility.DoDispatch (IVisualElementPanel panel) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:214)
UnityEngine.Experimental.UIElements.UIElementsUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at /Users/builduser/buildslave/unity/build/Runtime/UIElements/Managed/UIElementsUtility.cs:67)
UnityEngine.GUIUtility.ProcessEvent (Int32 instanceID, IntPtr nativeEventPtr) (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:186)

Workaround: press any of the buttons from the top row of macOS (next to Apple icon), for example on "File".