How to allow users change theme for specific pages
You hook_custom_theme()

function MYMODULE_custom_theme() {
  // Set path specific theme.
  if (current_path() == 'Your Path') {
    return 'Your theme name here';
  }
}


Note: Clear cache and have a check

Or You can use Drupal module: ThemeKey


  ThemeKey allows you to define simple or sophisticated theme-switching
  rules which allow automatic selection of a theme depending on current
  path, taxonomy terms, language, node-type, and many, many other
  properties.