build available theme list dynamically

app/routes/users.py

@@ -169,6 +169,9 @@ def get_prefers_theme():
     if not 'theme' in session:
         return 'style'
 
+    if session['theme'] not in current_app.config['allowed_themes']:
+        return 'style'
+
     return session['theme']
 
 @bp.get("/log_in")
@@ -294,8 +297,8 @@ def settings_form(username):
     # we silently ignore the passed username
     # and grab the correct user from the session
     user = get_active_user()
-    theme = request.form.get('theme', default='default')
-    if theme == 'default':
+    theme = request.form.get('theme', default='style')
+    if theme == 'style':
         if 'theme' in session:
             session.pop('theme')
     else: