* 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 base::FilePath& path) {
  if (path.empty()) {
    args->ThrowError("path cannot be empty");
    return;
  }
  DevToolsRemoveFileSystem(path);
}
void WebContents::Undo() {
  web_contents()->Undo();
}
void WebContents::Redo() {
  web_contents()->Redo();
}
void WebContents::Cut() {
  web_contents()->Cut();
}
void WebContents::Copy() {
  web_contents()->Copy();
}
void WebContents::Paste() {
  web_contents()->Paste();
}
void WebContents::PasteAndMatchStyle() {