There are a number of ways of clearing the registry.
In a stock Drupal installation, visiting admin/settings/performance (Home | Administer | Site conﬁguration | Performance) and clicking on the Clear cached data button will clear all cached data, including the registry, and force a rebuild.
As a result, any changes that affect the structure of the theme will necessitate the clearing of the theme registry.
Rebuilding the registry is an intensive operation which is required only when changes have been made to the theme's ﬁles.
The Devel module provides a Development block which can be enabled via the block management page at admin/build/blocks (Home | Administer | Site building | Blocks).
Custom themes are the equivalent of handcrafted pieces of art as the themer controls every piece of the puzzle from a design or implementational point of view.Additionally, the Zen theme comes with extensive documentation within each ﬁle which make things all the more convenient.With respect to CSS, Zen maintains a number of well documented CSS ﬁles segregated by functionality or location.A shortcut It is sometimes handy to know that the cache and registry can also be cleared by visiting admin/build/themes (Home | Administer | Site building | Themes) and just clicking the Save conﬁguration button.However, during development or debugging, we will want to clear the registry with great regularity.This includes setting up the theme using ﬁles, choosing the layout, implementing it in a page template, adding regions, styling nodes using node templates, blocks using block templates, and so on.