If you've found an existing theme or theme framework that you like and you just want to adjust it a bit to fit your requirements hand-in-glove, you can create a child theme on top of this existing theme. A child theme uses the parent theme as a starting point and, without changing the theme itself, alters just the bits you want to alter.
As a matter of fact, using child themes is the recommended way of making modifications to any theme. The rule of thumb is simple--if you want to change anything at all about a stock theme (either inside the source code, graphics, or template files), do it through a new child theme.
In plain English, a child theme inherits the functionality and features of the parent theme. The biggest value of creating child themes is that you can introduce any bells and whistles you wish without altering the structure of the parent...
