Why two templates?
Joomla uses two templates one is a default template for displaying your website in a particular way, calling upon various css files. This will govern placement and font types and colour etc. The second template used by Joomla is the admin template, which affects how the administration options are displayed to you.
The default install of Joomla will give you a couple of different options to choose from. Changing a template may produce some unexpected changes to your website. I would recommend that anyone considering using Joomla experiments with the sample data and templates before committing to building a website.
With each chosen template you can create various styles of that template. So you may wish to have less facilities for a particular user or group of users, you may wish to display different colours for a particular person or only give partial access to what is being offered by the template. You can assign different template styles to a user.
The reason I proffer this information is that on my first website I transferred into Joomla I changed the default template and thought I had lost most of my data, I hadn’t it just wasn’t being displayed in any meaningful way.
There are literally hundreds of templates available for Joomla and you are sure to find one that suits you best. You will still want to tweak some aspect eventually but as a time-saving device let the professionals do the basic design work.
Behind many of the purchased templates lies a system to easily manipulate the template. With Gantry used by Rocket themes you have a slider system to adjust columns or any other template component, making it easier for the ordinary user to effect style changes. Other template companies have their own take with the same end in mind e.g. http://www.yootheme.com have something called warp.