add barebones theme switcher

app/routes/users.py

@@ -165,6 +165,12 @@ def admin_only(*args, **kwargs):
     return decorator
 
 
+def get_prefers_theme():
+    if not 'theme' in session:
+        return 'style'
+
+    return session['theme']
+
 @bp.get("/log_in")
 @redirect_if_logged_in(".page", username = lambda: get_active_user().username)
 def log_in():
@@ -288,6 +294,12 @@ 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':
+        if 'theme' in session:
+            session.pop('theme')
+    else:
+        session['theme'] = theme
     topic_sort_by = request.form.get('topic_sort_by', default='activity')
     if topic_sort_by == 'activity' or topic_sort_by == 'thread':
         sort_by = session['sort_by'] = topic_sort_by