Improve contrast of buttons in post/release pages

Use the .btn-primary CSS class for buttons instead of .btn-secondary,
which makes them have blue background with white text. This greatly
improves contrast.

_includes/post.html

@@ -29,11 +29,11 @@ <h1 itemprop="headline">{{ title }}</h1>
    <time itemprop="dateCreated">{{ page.date | date: "%e %B %Y" }}</time>
  {% endif %}
   {% if download %}
-  <a class="btn btn-l btn-secondary mt-3" href="{{ download }}">Download</a>
+  <a class="btn btn-l btn-primary mt-3" href="{{ download }}">Download</a>
   {% elsif package and version %}
-  <a class="btn btn-l btn-secondary mt-3" href="{{ site.release_dir | append:'/' | append: package | append: '-' | append: version | append: '.tar.xz' }}">Download</a>
+  <a class="btn btn-l btn-primary mt-3" href="{{ site.release_dir | append:'/' | append: package | append: '-' | append: version | append: '.tar.xz' }}">Download</a>
     {%- if hasDocumentation and not hideDocumentation %}
-	<a class="btn btn-l btn-secondary mt-3" href="{{ site.api_ref_dir }}/{{ version }}/">Documentation</a>
+	<a class="btn btn-l btn-primary mt-3" href="{{ site.api_ref_dir }}/{{ version }}/">Documentation</a>
 	{%- endif %}
   {% endif %}
 </header>