build available theme list dynamically

app/templates/users/settings.html

@@ -17,8 +17,9 @@
   <form method='post'>
     <label for='theme'>Theme (beta)</label>
     <select autocomplete='off' id='theme' name='theme'>
-      <option value='default' {{ 'selected' if get_prefers_theme() == 'style' }}>Default</option>
-      <option value='theme-otomotone' {{ 'selected' if get_prefers_theme() == 'theme-otomotone' }}>Otomotone (beta)</option>
+      {% for theme in config.allowed_themes %}
+        <option value="{{ theme }}" {{ 'selected' if get_prefers_theme() == theme }}>{{ theme | theme_name }}</option>
+      {% endfor %}
     </select>
     <label for='topic_sort_by'>Sort threads by:</label>
     <select id='topic_sort_by' name='topic_sort_by'>