* 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
web_contents()->PasteAndMatchStyle();
}
void WebContents::Delete() {
  web_contents()->Delete();
}
void WebContents::SelectAll() {
  web_contents()->SelectAll();
}
void WebContents::Unselect() {
  web_contents()->CollapseSelection();
}
void WebContents::Replace(const base::string16& word) {
  web_contents()->Replace(word);
}
void WebContents::ReplaceMisspelling(const base::string16& word) {
  web_contents()->ReplaceMisspelling(word);
}
uint32_t WebContents::FindInPage(mate::Arguments* args) {
  uint32_t request_id = GetNextRequestId();
  base::string16 search_text;
  blink::WebFindOptions options;
  if (!args->GetNext(&search_text) || search_text.empty()) {
    args->ThrowError("Must provide a non-empty search content");
    return 0;
  }
  args->GetNext(&options);
  web_contents()->Find(request_id, search_text, options);
  return request_id;
}
void WebContents::StopFindInPage(content::StopFindAction action) {