The first question you need to ask yourself is: Do I need to customize the styling, or do I need to customize the structure? (Of course there is always the chance that the answer to both is 'yes.') We pose this question as it frames the next steps: If you only need to customize the styling, then you just need to look at working with the theme's CSS. Assuming that adequate selectors are already in place in the code, this means simply overriding some of the system's many styles. If, on the other hand, you need to customize the structure, then you are likely needed to override templates or themable functions.
Note
Note that the Drupal theme system caches template files, theme functions, and .info files. Accordingly, as you work on the theme and make changes to any of those elements, you should refresh Drupal's cache in order to see your change take effect immediately.
The process of customizing a theme into something new consists of a set of tasks that can be categorized...