Can’t get jquery script to work

It is becuase you didn’t enqueue jquery, replce your mytheme_custom_scripts() function with below code.

function mytheme_custom_scripts(){

    // include jquery
    wp_enqueue_script('jquery');

    wp_enqueue_script( 'my-test-script', get_template_directory_uri() . '/js/my-jquery-script.js', array('jquery'), '1.0', true );

}

and jQuery code like this

jQuery(document).ready(function($){
  $('h1, h2, h3, h4, h5, h6, p, a').css('color','pink');
});