Replace deprecated style key in HTML template

`style` superceded by `styles` in Sphinx 5.1 and removed in Sphinx 7
sphinx-doc/sphinx#11381

Author: guyer

documentation/_themes/nist/basic_layout.html

@@ -94,7 +94,7 @@ <h3>{{ _('Navigation') }}</h3>
 {%- endmacro %}
 
 {%- macro css() %}
-    <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
+    <link rel="stylesheet" href="{{ pathto('_static/' + styles[-1], 1) }}" type="text/css" />
     <link rel="stylesheet" href="{{ pathto('_static/pygments.css', 1) }}" type="text/css" />
     {%- for css in css_files %}
       {%- if css|attr("filename") %}