In a new, clean Magento Community installation, you are provided with the following design packages and themes:

Depending on your needs, you could add additional custom design packages, or custom themes within the Magento design package:
We like to make sure that whatever we customize can be "undone" if necessary. It's difficult for us to make changes to the core, installed files; we prefer to work on duplicate copies, preserving the originals in case we need to revert back. After re-installing Magento for the umpteenth time because we had altered too many core files, we learned this the hard way!
Your new theme hierarchy might now look like this:

If you decide to use one of the installed default themes as the basis for designing a new, custom theme, duplicate and rename the theme to preserve the original as your fallback.
The Blank Theme
A default installed theme is called Blank. If the customization to your Magento stores is primarily one of colors and graphics, this is not a bad theme to use as a starting point. As the name implies, it's a pretty stark layout, as shown in the following screenshot. However, it does give you all the basic structures and components.

Stark layout