Question: I would like to modify my WordPress theme without changing the code. Specifically, I would like to show the number of minutes required to read the post, How can I do that?
Similare to showing posting date and author.

Answer: 
Go to your Dashboard>Appearance>Themes>Customize

Make sure to install theme that has more than standard options.
You can install a plugin as per: <https://www.shoutmeloud.com/estimated-reading-time-wordpress-posts.html>

There is a full installation and configuration tutorial here and you will get your desired result of showing how many minutes it will take to a read a post.