Variable {$ css_dir} {$ js_dir} not defined in Prestashop

Viewing the documentation to create themes, use the variables {$css_dir} / {$js_dir} to be able to access the corresponding folders of the theme.

I have the folders created and I try to use the variables in the following way in my head.tpl

{block name='hook_header'}
  {$HOOK_HEADER nofilter}
  <link href="//fonts.googleapis.com/css?family=Source+Sans+Pro" rel="stylesheet" />
  <link rel="stylesheet" href="{$css_dir}icons/estilos.css" />
{/block}

But he always tells me that the variable does not exist and gives me an error. Should we put it in some other way? It happens to me trying to use both the css and the js. The version I'm working with is Prestashop 1.7