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.
Let's say you write a lot about cheese and gadgets. Through the 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 currently comes with four themes: the default Twenty Fifteen theme, and previous defaults Twenty Fourteen theme, Twenty Thirteen theme and Twenty Twelve theme. You can switch between Themes using the Appearance admin panel. Themes that you add to the theme directory will appear in the Administration Screen > Appearance > Themes as additional selections.
There are many Themes available for download that will work with your WordPress installation.
If the Theme that you are installing provides instructions, be sure to read through and follow those instructions for the successful installation of the Theme. It is recommended that Theme developers provide installation instructions for their own Themes, because Themes can provide special optional functionality that may require more steps than the basic installation steps covered here. If your Theme does not work after following any provided instructions, please contact the Theme author for help.
You can download Themes directly to your blog by using the Add New Themes option in the Appearance sub-menu.
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 containing a folder under which all the Theme files reside.
To add a new Theme to your WordPress installation, follow these basic steps:
To select a Theme for your site:
Your selection will immediately become active.
Note: If the Theme preview is blank, do not activate the new Theme without investigating further. Your site may not be displayed correctly, otherwise.
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.
If you simply want to customize your current Theme for your own use, consider creating a Child Theme.