A lot of commercial templates use all manner of tweaks and cheats to get the look them have in a demo.
I say cheats because you expect certain display items to be replicated easily but you find out they require the addition of inline CSS (inserted without the use of WYSIWYG editors, because the code is not valid!) &etc &etc.
Also, RocketTheme templates are very complex and usually need many of their extensions to be separately installed, on the Iridium theme there are 10 of these RocketTheme extensions. You can see all this on the live demo site, under the 'Extensions' menu item:
http://demo.rockettheme.com/live/joomla/iridium/
On this template I would assume the modules have to contain Class names and the template CSS will style them correctly, for example:
- - In the Module edit screen > Advanced Tab > you will find ' Module Class Suffix'
- Using Firebug, look at the CSS of the live version of the demo site and see if these modules have extra Class names.
There is usually a lot of documentation for each template, start here:
http://demo.rockettheme.com/live/joomla ... stallation
As
jamman14 has pointed out install the the full demo version on your site and remove the things you don't need.
You may find that is a bit messy and time consuming, so what I usually do is install the full demo version locally or on a sub-directory of my website. Then I have some thing to compare against and investigate!