Magento’s theme system is the most complex system. It is full of trap, trick…… But after reading all the source code of Magento, also can understand the Magento theme system and can easy customize the theme.

A Magento theme consists of any or all of the following:
Layout
app/design/frontend/your_interface/your_theme/layout/
Templates
app/design/frontend/your_interface/your_theme/template/
Locale
app/design/frontend/your_interface/your_theme/locale/
Skins
skin/frontend/your_interface/your_theme/

Most Important Files
app/design/frontend/your_interface/your_theme/layout/page.xml : default layout for all pages

Enable Template Hints
The template hints tell us where to find the layouts, enable it this way:
Admin -> System -> Configuration -> set the configuration scope to the website -> ADVANCED -> Developer -> Debug -> Template Path Hints

Using the New Theme
Admin -> System -> Configuration -> set the configuration scope to the website -> Design -> Themes -> Default

Disable system cache

The system cache can be disabled at Administration Panel -> System -> Cache Management.

Links

Add Links block:

<block type="page/template_links" name="top.links" as="topLinks"/>

Add link items:

<action method="addLink" translate="label"><label>Home</label><url>/</url></action>