from flask_assets import Bundle, Environment
from flask import current_app

bundles = {
  'root_js': Bundle(
    'js/lib/jquery-2.1.1.min.js',
    'js/app.js',
    output='dist/bundle.js'),
  
  'root_css': Bundle(
    'css/lib/reset.css',
    'css/common.css',
    'css/app.css',
    output='dist/styles.css')
}

@@ -67,16 +67,23 @@
 rent_app
+ as app
 %0A%0Abundle
@@ -313,8 +313,60 @@
 css')%0A%7D%0A
+%0Aassets = Environment(app)%0Aassets.register(bundles)%0A

Update assets.py