The solution (or problem) is not limited to the 2017 theme. This is something you can add to any theme. Since it involves changing the functions.php file, you should first make a Child Theme of the 2017 theme. (Ask the googles how to do that….and why you would use a Child Theme rather than changing the theme code.)
In fact, if you had asked the googles the same question, the first result would have been the Codex, where all is revealed. https://codex.wordpress.org/Customizing_the_Read_More .