Account for Sphinx 4 changes in pygments CSS files

Sphinx 4 changes the handling of pygments-related CSS files, including
the .css file directly in `css_files`. This breaks dark mode syntax
highlighting since pygments.css file is now included twice -- once
before the pygments_dark.css and once after.

This break the whole mechanism, since the highlighting is sentitive to
the ordering of those files being included, due to how Sphinx generates
those files.

This change works around the issue, by adding the pygments_dark.css file
after both the pygments.css files.

src/furo/theme/furo/base.html

@@ -54,7 +54,6 @@
     {%- block theme_styles -%}
     <link rel="stylesheet" href="{{ pathto(furo_assets['style'], 1) }}">
     <link rel="stylesheet" href="{{ pathto('_static/pygments.css', 1) }}">
-    <link media="(prefers-color-scheme: dark)" rel="stylesheet" href="{{ pathto('_static/pygments_dark.css', 1) }}">
     {% include "partials/_head_css_variables.html" with context %}
     {%- endblock -%}
 
@@ -68,6 +67,7 @@
     {%- endblock regular_styles -%}
 
     {%- block extra_styles -%}
+    <link media="(prefers-color-scheme: dark)" rel="stylesheet" href="{{ pathto('_static/pygments_dark.css', 1) }}">
     <link rel="stylesheet" href="{{ pathto(furo_assets['furo-extensions.css'], 1) }}">
     {%- endblock -%}