There are several tools that that makes designing for Drupal much, much easier.
In particular, we recommend the Theme developer module and also Firebug.
Think of Theme developer as a Drupal-specific version of Firebug. Using Theme developer you can click on any element of your Drupal site and get a breakdown of how it was built.
This video is part of complete Drupal theming video class.
Note: please make sure to install and older version of the simplehtmldom API module. You need version 7.x-1.12, not the latest release.