Two mainstay modules to use in theme development are the Devel modules and the Theme Developer module.

Devel modules

This package of modules was created for site developers to aid in site development. Two handy features are:

  1. Rebuild theme registry – This forces the theme registry to be rebuilt on every pageload. Without this setting enabled, changes and additions to theme templates will not be read without clearing caches or visiting the Appearance page (Drupal 7). Note that this results in a significant performance hit on your site; as a result, it should never be used on a live site!
  2. Create temporary content – This submodule allows you to quickly create nodes, comments, taxonomy terms and users to provide temporary content to aid in your theming efforts.

Theme Developer module

This module provides, via overlay, insight into Drupal functions and content in the theming layer.