blank theme : How-to ?

Starting from a blank theme is fine if you'd like to do it that way. Note that there will still be some core CSS files loaded, but you can override those in your theme so they don't get loaded at all if you prefer.

As far as the responsive menu, you can disable that without removing the core CSS completely as well -- look in the block settings on the Layout in question, and uncheck the option to display "Display menu toggle button on small screens":

To disable in your theme template.php file, add the following hooks.  I'm not sure if these are the correct ones for your use case, but its a good place to start.

/**
 * Implements hook_css_alter().
 */
function custom_theme_css_alter(&$css) {
  unset($css['core/modules/system/css/menu-dropdown.theme.css']);
  unset($css['core/modules/system/css/menu-toggle.theme.css']);
  unset($css['core/misc/smartmenus/css/sm-core-css.css']);
  unset($css['core/modules/system/css/menu-dropdown.theme.breakpoint.css']);
  unset($css['core/modules/system/css/menu-dropdown.theme.breakpoint-queries.css']);
  unset($css['core/misc/normalize.css']);
  unset($css['core/modules/system/css/system.css']);
  unset($css['core/modules/system/css/system.theme.css']);
  unset($css['core/modules/layout/css/grid-flex.css']);
  unset($css[backdrop_get_path('module', 'system') . '/system.messages.css']);
  unset($css[backdrop_get_path('module', 'system') . '/system.theme.css']);
  unset($css[backdrop_get_path('module', 'system') . '/system.menus.css']);
}

/**
 * Implements hook_js_alter().
 */
function custom_theme_js_alter(&$javascript) {
  unset($javascript['core/modules/layout/js/grid-fallback.js']);
  unset($javascript['core/modules/system/js/menus.js']);
  unset($javascript['core/misc/smartmenus/jquery.smartmenus.combined.js']);
  unset($javascript['core/misc/smartmenus/jquery.smartmenus.js']);
  unset($javascript['core/modules/contextual/js/contextual.js']);
}