* 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
const gfx::Rect& pos) {
  Emit("move", pos);
}
void WebContents::CloseContents(content::WebContents* source) {
  Emit("close");
  if ((type_ == BROWSER_WINDOW || type_ == OFF_SCREEN) && owner_window())
    owner_window()->CloseContents(source);
}
void WebContents::ActivateContents(content::WebContents* source) {
  Emit("activate");
}
void WebContents::UpdateTargetURL(content::WebContents* source,
                                  const GURL& url) {
  Emit("update-target-url", url);
}
bool WebContents::IsPopupOrPanel(const content::WebContents* source) const {
  return type_ == BROWSER_WINDOW;
}
void WebContents::HandleKeyboardEvent(
    content::WebContents* source,
    const content::NativeWebKeyboardEvent& event) {
  if (type_ == WEB_VIEW && embedder_) {
    
    embedder_->HandleKeyboardEvent(source, event);
  } else {
    
    CommonWebContentsDelegate::HandleKeyboardEvent(source, event);
  }
}
content::KeyboardEventProcessingResult WebContents::PreHandleKeyboardEvent(