* 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 "content/public/browser/navigation_details.h"
#include "content/public/browser/navigation_entry.h"
#include "content/public/browser/navigation_handle.h"
#include "content/public/browser/plugin_service.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/render_widget_host.h"
#include "content/public/browser/render_widget_host_view.h"
#include "content/public/browser/resource_request_details.h"
#include "content/public/browser/service_worker_context.h"
#include "content/public/browser/site_instance.h"
#include "content/public/browser/storage_partition.h"
#include "content/public/browser/web_contents.h"
#include "content/public/common/context_menu_params.h"
#include "native_mate/converter.h"
#include "native_mate/dictionary.h"
#include "native_mate/object_template_builder.h"
#include "net/url_request/url_request_context.h"
#include "third_party/WebKit/public/platform/WebInputEvent.h"
#include "third_party/WebKit/public/web/WebFindOptions.h"
#include "ui/display/screen.h"
#include "ui/events/base_event_utils.h"
#include "ui/latency/latency_info.h"
#if !defined(OS_MACOSX)
#include "ui/aura/window.h"
#endif
#include "atom/common/node_includes.h"
namespace {
struct PrintSettings {