Before the advent of Themes, WordPress generated content using a single file, index.php, and files to support comment display and submission. A single style sheet controlled the presentation. All other pages, including the category and archive pages, were generated by passing parameters to the index.php page.
The new Theme system provides two convenient features.
The old method of generating pages will still work. If you are upgrading from v1.2 or v1.2.1 or v1.2.2, you can continue using your existing design. Keeping your older design will not prevent you from adding additional themes and easily switching between designs.
Fundamentally, the WordPress Theme system is a way to "skin" your weblog. Yet, it is more than just a "skin." Skinning your site implies that only the design is changed. WordPress Themes can provide much more control over the look and presentation of the material on your website.
A WordPress Theme is a collection of files that work together to produce a graphical interface with an underlying unifying design for a weblog. These files are called template files. A theme modifies the way the site is displayed, without modifying the underlying software. Themes may include customized template files, image files (*.jpg, *.gif), style sheets (*.css), custom Pages, as well as any necessary code files (*.php). For an introduction to template files, see Stepping Into Templates.
Themes are a whole new ball game. Let's say you write a lot about cheese and gadgets. Through the innovative use of the WordPress Loop and template files, you can customize your Cheese category posts to look different from your Gadgets category posts. With this powerful control over what different pages and categories look like on your site, you are limited only by your imagination. For information on how to use different Themes for different categories or posts, see The Loop in Action and Category Templates.
The WordPress Theme Directory is the official site for WordPress Themes which have been checked and inspected, and are free for downloading. The site features the ability to search by type and style, and offers a demonstration of the page view elements of the Theme.
WordPress supplies two themes in its distribution for your initial use. You can switch between these two themes using the admin panel. Themes that you add to that directory will appear in the Administration Panels > Design > Themes as additional selections.
There are many themes available for download that will work with your WordPress installation.
To add a new theme to your WordPress installation, follow these basic steps:
If your host offers the cPanel control panel, and the theme files are in a .zip or .gz archive follow these instructions. Note: This assumes the theme you download is a compressed (zip) file and the files in the zip file are in their 'named' folder.
Note: You can also install and activate the plugin Get_Theme, to download themes directly to your blog.
To select the active theme for your site:
Your selection should immediately become active.
If you are interested in creating your own theme for distribution, or learning more about the architecture of themes, please review the documentation regarding Theme Development and Designing Themes for Public_Release.
The following are the files typically included within a Theme.
== Moving from 1.2.x to 1.5 ==
For details on upgrading WordPress and your WordPress Theme from v1.2 to 1.5, see: Upgrade 1.2 to 1.5.
These tools and resources will aid you in creating and/or enhancing themes.