Fix AttributeError when one of css_files is a string (#1537)

Fix AttributeError when one of css_files is a string

Author: mitya57

sphinx_rtd_theme/layout.html

@@ -29,7 +29,11 @@
     <link rel="stylesheet" href="{{ pathto('_static/pygments.css', 1) }}" type="text/css" />
   {%- endif %}
   {%- for css_file in css_files %}
-    {{ css_tag(css_file) }}
+    {%- if css_file|attr("filename") %}
+      {{ css_tag(css_file) }}
+    {%- else %}
+      <link rel="stylesheet" href="{{ pathto(css_file, 1)|escape }}" type="text/css" />
+    {%- endif %}
   {%- endfor %}
 
   {# "extra_css_files" is a theme option and it's always a string #}