To accurately preview the site, you need to set up apache + php + mysql on your local PC.
If you edit the theme files directly in Dreamweaver or another editor, you can change the html and css around and get a limited preview by using the "server" in Dreamweaver, but to really see an accurate rendering of your site you have to run it using apache, just like on your real hosting.
Editing minor stuff like colours is easy, but if you really want to start hacking around with the layout then a small amount of PHP knowledge is usually required.
Luckily it's relatively easy to set up apache / php / mysql on your local machine. For Windows there are easy installation packages like:
http://www.wampserver.com/en/
http://www.easyphp.org/
http://www.apachefriends.org/en/xampp-windows.html
If you're using a Mac then from memory everything you need comes preinstalled.
Of course, you could keep on uploading your changes to your hosting account every time you make a change, but this is a real pain in the ass, plus you could break your site temporarily which is no good if that's when Googlebot decides to swing by.
Keep a backup of your original theme files!