old_contents: from flask import Flask, render_template
  app = Flask(__name__, static_url_path="", static_folder="src")
  
  
  @app.route("/")
  def index():
      return render_template("index.html")
  
  if __name__ == "__main__":
      app.run(debug=True)
  
new_contents: from flask import Flask, render_template
  from sassutils.wsgi import SassMiddleware
  app = Flask(__name__, static_url_path="", static_folder="src")
  app.wsgi_app = SassMiddleware(app.wsgi_app, {
      'src': ('scss', 'css', '/css')
  })
  
  @app.route("/")
  def index():
      return render_template("index.html")
  
  if __name__ == "__main__":
      app.run(debug=True)
  
subject: Add SassMiddleware