Okay what I suggest you do to start with is go to Web\App_Themes take a copy of the dashCommerce directory and then paste it into the Web\App_Themes directory and name the diretory whatever you like.
If you then login to the admin control goto site settings > general there will be a drop downlist called theme. You can pull down on the drop and select the directory you created in the step above. This way if you screw anything up you can just copy the dashCommerce directory and restart, good way for learning.
All pages use mastepages you can alter Site.Master to make dramatic changes if need be.
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