CSS - Cascading Style Sheets is a web standard that controls the design layout (Look & Feel) of a website, content and graphics. It has many advantages over traditional designs. One important one is the separation of the design of a website and the functionality. You can quickly change the design to a completely different look and feel without affecting the core functions of the website, like the text content, login, menus...etc. What will change is how they look, not how they work. Other advantages are that it is dynamically driven, faster, smaller file sizes and cached saved for faster browser reloads.
Joomla CMS uses CSS and this allows for 3rd party development and designs of templates that are free or commercially available to be customized to your needs. The only downsides are; the need for designers to be familiar with designing using CSS and compliant browser versions. CSS is different from the traditional designs, but is gaining in acceptance because of the reasons listed.
One advantage of using templates, is the speed of deployment and costs. Their are many templates available and most only need minor changes or tweekings to give you what you need. With the CMS platform, adding extensions, components, modules and plugins will be embedded in the functionality and incorporate the design without making large design changes, some even have their own CSS to customize even more.