Configuring static files (CSS) in Java11 Appengine project

you need to change handler url of webapp. it has to be like this:

handlers:
  - url: /webapp/(.*\.(gif|png|jpg|js|css|json))$
    static_dir: webapp\1

It uses regex for those files resides in nested folders

handlers:
# All URLs beginning with /stylesheets are treated as paths to
# static files in the stylesheets/ directory.
- url: /stylesheets
  static_dir: stylesheets
  # ...

Picked from documentation