* Electron version: 1.7.5

### Expected behavior

When a theme color is no longer present the webContents did-change-theme-color event should return `null`.

### Actual behavior

The event returns `"#000000"`.

### How to reproduce

1. Log the returned color of the webContents did-change-theme-color event.
2. Go to https://electron.atom.io which has a theme color.
3. Go to https://atom.io which does not have a theme color.




atom/browser/api/atom_api_web_contents.cc
#include "atom/browser/api/atom_api_web_contents.h"
#include <set>
#include <string>
#include "atom/browser/api/atom_api_debugger.h"
#include "atom/browser/api/atom_api_session.h"
#include "atom/browser/api/atom_api_window.h"
#include "atom/browser/atom_browser_client.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/atom_browser_main_parts.h"
#include "atom/browser/atom_javascript_dialog_manager.h"
#include "atom/browser/child_web_contents_tracker.h"
#include "atom/browser/lib/bluetooth_chooser.h"
#include "atom/browser/native_window.h"
#include "atom/browser/net/atom_network_delegate.h"
#if defined(ENABLE_OSR)
#include "atom/browser/osr/osr_output_device.h"
#include "atom/browser/osr/osr_render_widget_host_view.h"
#include "atom/browser/osr/osr_web_contents_view.h"
#endif
#include "atom/browser/ui/drag_util.h"
#include "atom/browser/web_contents_permission_helper.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/browser/web_contents_zoom_controller.h"
#include "atom/browser/web_view_guest_delegate.h"
#include "atom/common/api/api_messages.h"