There is one module that makes designing for Drupal 7 much, much easier: Theme Developer.
You can think of Theme Developer as a Drupal-specific version of Firebug or Chrome Developer Tools. Using Theme developer you can click on any element of your Drupal site and get a breakdown of how it was built.
Theme Developer has some downsides: it's not been updated in a while, and (like anything related to the Devel module) shouldn't be used on live sites. But, it can still be a useful tool for Drupal 7 themers.