First, you need to make a new directory for your custom template files.
Open: includes/templates/ and add a folder. (You may call it anything you want, but in this FAQ we will be calling it my_template.)
Create an empty folder inside your new template directory and call it images ... so you should have includes/ templates/ my_template/ images/.
Next, copy the includes/ templates/ template_default/ css directory from includes/ templates/ template_default/ and place the new folder and its files in your new includes/ templates/ my_template/ folder.
Copy the stylesheet.css from includes/ templates/ template_default/ css to includes/ templates/ my_template/ css. This si the stylesheet you will be editing.