CSS templates are websites made by div coding. It's new technology. Many years ago was table coding. It;s was not so good, coding was hard and on editing of text and styles spend more times.
CSS templates use as a basic div element (conteiner, box) with spesific classes. In external file there are styles for appropriate class and if you need to change for example color of all links you easy can do it in one place and modification will apply for all site.
Now CSS coding is a standart type of any our website templates. Using such technology makes site more light weight and loading of page is decreased. It's also makes it more SEO friendly - rendering of the page more understandable for search engines. And the main goal that customization and editing is very simple, styles of text and blocks more sorted and can be changed just in few steps.