When you want to customize my PT004 theme please read this page.
When you want to make your own changes. Make sure you do not change the default theme files. If you do they will be overwritten when you update the theme. Below you can see how to make your changes in css. You should use the sample-pt004-custom.css file, as you can see below and rename it to pt004-custom.css than add your changes in this file.
Create your own sub theme