script.js

jQuery(function($) {
  var beforeButton = $("<i class='incr-button fa fa-minus-circle fa-2x' />");
	var beforeButtonClick = function() {
		var input = $(this).next();
		input.val(new Number(input.val()) - 1);
	}
	var afterButton = $("<i class='incr-button fa fa-plus-circle fa-2x' />");
	var afterButtonClick = function() {
		var input = $(this).prev();
		input.val(new Number(input.val()) + 1);
	}
	$("input[type='number']").before(beforeButton);
	$("input[type='number']").after(afterButton);
  
})

style.css

/* Styles for buttons */

.fa-plus-circle {
  color: green
}

.fa-minus-circle {
  color: green
}

wp-number-input-buttons.php

<?php
/** 
* Plugin Name: Buttons on my Number Inputs
* Author: Liam Bailey (Webby Scots)
* Description: Adds nice Font Awesome buttons to number inputs on the site
* Author URI: https://webbyscots.com
**/

add_action('wp_enqueue_scripts', 'wswp_add_scripts');

function wswp_add_scripts() {
	wp_enqueue_script('font-awesome', 'https://use.fontawesome.com/9f67fc414e.js');
	wp_enqueue_style('my-plugin-css', plugins_url('style.css', __FILE__), NULL, time());
   	wp_enqueue_script('my-plugin-js', plugins_url('script.js',__FILE__), array('jquery'), time());
}