This is the short version:
CSS offers a great deal, IF and only if you understand how to use it and how (and why) it works.
Most people assume that CSS is for general formatting of page elements like font style, size, colour, background and so on.
This is certainly true, but good CSS pages and layouts also have their positioning controlled by the external CSS file, like floating an image to one or another side and wrapping text around it, or producing text columns of just such a wdith and so on.
At their best, CSS files allow a potentially huge amount of flexibility in sitewide alterations (nice on a large site where one file may be edited to produce a global change) and in page load speeds and accessibility.
However, and this is the biggest consideration anyone should be aware of, web browsers are pants at interpreting CSS and not one of them gets it right all of the time even on CSS1, never mind the current CSS2 standard or the up and coming CSS3.
In plain English this means you have a great deal of potential benefits (emphasis on potential) but an equal if not larger number of likely pitfalls and everything you produce should be very carefully tested in all major web browsers.
I dislike templates in general from a fundamental point of view - they offer no real advantages to anyone willing to spend a bit of time learning how to produce their own output, and you may find yourself using the same layout as anyone else who bought and/or downloaded it.
In the case of CSS, unless the template is very simple or comes with full code commenting to tell you what goes where, why and what it does while it's there, you'll be no further forward since you won't understand how it works and so you won't understand how to use and change things in the long term. You will effectively be filling in the blank page areas with your own text and images, hopefully without breaking the layout in the process.
I really like CSS but it takes a degree of effort and time to gain enough of a handle on it to implement it effectively.
Rather than fool around with a template you'd be better off learning about it and having the freedom to design and roll out your own layouts. It will certainly be of more benefit to you.
If you'd like some links to CSS resources, feel free to ask. There are some truly spectacular sites out there that dedicate their content to CSS layouts.