feat: Use primary color specified in config

Sorry, not sorry iPhone users, get a phone on which you are able to use something different than Safari, or get a newer one :)

templates/includes/head.html

@@ -18,6 +18,13 @@
     {% endfor %}
   {% endif %}
 
+  <style>
+    :root {
+      --primary-color-alpha: {{ config.extra.primary_color_alpha }};
+      --primary-color: {{ config.extra.primary_color }};
+    }
+  </style>
+
   {% if config.extra.animated_favicon %}
     <link rel="icon" type="image/gif" href="{{ get_url(path='favicon.gif') }}" />
   {% else %}