Use css_tag helper to inject CSS files (#1519)

* Use `css_tag` helper to inject CSS files

* Typo

Author: humitos

sphinx_rtd_theme/layout.html

@@ -29,15 +29,11 @@
     <link rel="stylesheet" href="{{ pathto('_static/pygments.css', 1) }}" type="text/css" />
   {%- endif %}
   {%- for css in css_files %}
-    {%- if css|attr("rel") %}
-      <link rel="{{ css.rel }}" href="{{ pathto(css.filename, 1) }}" type="text/css"{% if css.title is not none %} title="{{ css.title }}"{% endif %} />
-    {%- else %}
-      <link rel="stylesheet" href="{{ pathto(css, 1) }}" type="text/css" />
-    {%- endif %}
+    {{ css_tag(css) }}
   {%- endfor %}
 
   {%- for cssfile in extra_css_files %}
-    <link rel="stylesheet" href="{{ pathto(cssfile, 1) }}" type="text/css" />
+    {{ css_tag(cssfile) }}
   {%- endfor -%}
 
   {#- FAVICON