* 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
content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  if (event.GetType() == blink::WebInputEvent::Type::kRawKeyDown ||
      event.GetType() == blink::WebInputEvent::Type::kKeyUp) {
    bool prevent_default = Emit("before-input-event", event);
    if (prevent_default) {
      return content::KeyboardEventProcessingResult::HANDLED;
    }
  }
  return content::KeyboardEventProcessingResult::NOT_HANDLED;
}
void WebContents::EnterFullscreenModeForTab(content::WebContents* source,
                                            const GURL& origin) {
  auto permission_helper =
      WebContentsPermissionHelper::FromWebContents(source);
  auto callback = base::Bind(&WebContents::OnEnterFullscreenModeForTab,
                             base::Unretained(this), source, origin);
  permission_helper->RequestFullscreenPermission(callback);
}
void WebContents::OnEnterFullscreenModeForTab(content::WebContents* source,
                                              const GURL& origin,
                                              bool allowed) {
  if (!allowed)
    return;
  CommonWebContentsDelegate::EnterFullscreenModeForTab(source, origin);
  Emit("enter-html-full-screen");
}
void WebContents::ExitFullscreenModeForTab(content::WebContents* source) {