don't hardcode styles to present, use config

now that we can override the styles in practice, we also need to only
present what is possible in the HTML

incorporealcms/templates/base.html

@@ -20,9 +20,9 @@
             {% endfor %}
         </div>
         <div class="styles">
-            <a href="?style=dark">[dark]</a>
-            <a href="?style=light">[light]</a>
-            <a href="?style=plain">[plain]</a>
+            {% for style in page_styles %}
+            <a href="?style={{ style }}">[{{ style }}]</a>
+            {% endfor %}
         </div>
     </div>
     {% endblock %}