Modify theme file within public app using api

Shopify Apps
1

As I seen there are no scope found to directly add/edit theme’s file within the Shopify public app.

NOTE: This is the public app

What I want to do?

  1. I want to add two files in theme when application is install
  • Snippets >> custom-cart.liquid
  • Templates >> custom-cart.meta.json.liquid
  1. I want to add my own class in theme where Checkout button is exists
  • Eg.: In cart page there are checkout button. I want to add class “custom-checkout-btn”

Please help me if anyone have solution for this

Thanks in advanced