Fundamentally, the WordPress Theme system is a way to “skin” your WordPress site. 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 visual presentation of your content and other data on your WordPress site, as well as behavior of certain site’s elements while interacting with visitors.
A WordPress Theme is a collection of files that work together to produce a graphical interface with an underlying unifying design for a website. 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, *.png, *.gif), style sheets (*.css), custom Pages, as well as any necessary code files (*.php). For an introduction to template files, see Template Files.
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 create custom look for various templates in your theme, take a look at Template Hierarchy.
The WordPress Theme Directory is the official site for WordPress Themes. Every theme in this directory is reviewed by a dedicated team and tested against wide range of rules, all of which are ensuring secure and pleasant experience for theme user.
With over 9,000 themes in the directory, you can easily find the right one for your site with advanced search feature using filters for layout, subject and specific theme’s features. This includes finding 
If you need more than a screenshot and features list to decide whether the theme fits your needs, you can see the demo for the theme with Preview feature on theme’s info page.
Most of themes in WordPress Theme Directory come with installation instructions, especially the ones that may require more steps than the usual theme installation. Be sure to read through and follow those instructions for the successful installation of the Theme. If your Theme does not work after following any provided instructions, please contact the Theme author for help.
A block theme is a theme that uses blocks for all parts of a site, including navigation menus, header, content, and site footer. These themes are built for the newest features coming to WordPress that allow you to edit and customize all parts of your site.
Currently, you can add block themes in the same way you would any other theme. However, the Live Preview option is removed which means you will need to set up a test site first to explore the theme before activating if you wish to preview it. You also will not have the option to use the Customizer since block themes rely on the Site Editor to edit all parts of your site. In order to customize a block theme, you’ll need to activate it first.
You can download any theme from WordPress Theme Directory directly to your site by using the Add New option in the Appearance sub-menu.
When the Theme is already downloaded but not activated Live Preview option will give you a preview of your site with your own, existing content.
If your host offers the cPanel control panel, you can use its Upload option to upload the Theme files to your site. For this you will need Theme files in an compressed archive (.zip or .gz).
To add a new Theme to your WordPress installation via FTP protocol, you’ll need FTP client and extracted Theme files.
Now that new Theme is in /wp-content/themes/ directory (whether you used Administration Screens, cPanel or FTP method), this new Theme is ready to be activated. All themes in /wp-content/themes/ directory are available for Activation and Update (when update is provided by theme author), but only one theme from this directory can be Active.
When theme is Activated it means that this theme’s style and functionality (look and behavior) will be applied on your site. You will be informed by Administration notification about successful activation of the Theme.
To activate a Theme for your site:
Log in to the WordPress Administration Screens.
Select the Appearance screen, then Themes.
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 do not see Theme’s thumbnail at all, your new Theme might be corrupted or broken. Take a look below installed theme’s thumbnails if there is any info about broken themes. In this case contact the Theme author for help.
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.
Whichever the case, you are welcome to join the Themes Team and their dedicated #themereview Slack channel. In that channel, you can ask for help on developing themes for WordPress Theme Directory or even start reviewing themes yourself.
