* 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/common/api/event_emitter_caller.h"
#include "atom/common/color_util.h"
#include "atom/common/mouse_util.h"
#include "atom/common/native_mate_converters/blink_converter.h"
#include "atom/common/native_mate_converters/callback.h"
#include "atom/common/native_mate_converters/content_converter.h"
#include "atom/common/native_mate_converters/file_path_converter.h"
#include "atom/common/native_mate_converters/gfx_converter.h"
#include "atom/common/native_mate_converters/gurl_converter.h"
#include "atom/common/native_mate_converters/image_converter.h"
#include "atom/common/native_mate_converters/net_converter.h"
#include "atom/common/native_mate_converters/string16_converter.h"
#include "atom/common/native_mate_converters/value_converter.h"
#include "atom/common/options_switches.h"
#include "base/process/process_handle.h"
#include "base/strings/utf_string_conversions.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/values.h"
#include "brightray/browser/inspectable_web_contents.h"
#include "brightray/browser/inspectable_web_contents_view.h"
#include "chrome/browser/printing/print_preview_message_handler.h"
#include "chrome/browser/printing/print_view_manager_basic.h"
#include "chrome/browser/ssl/security_state_tab_helper.h"
#include "content/browser/renderer_host/render_widget_host_impl.h"
#include "content/browser/renderer_host/render_widget_host_view_base.h"
#include "content/browser/web_contents/web_contents_impl.h"
#include "content/common/view_messages.h"
#include "content/public/browser/child_process_security_policy.h"
#include "content/public/browser/favicon_status.h"
#include "content/public/browser/native_web_keyboard_event.h"