In WordPress terminology, a theme is a premade graphical user interface by which visitors interact with a website and its content. A theme defines the structure, look and feel of the website on the front end.
WordPress themes define where posts and pages should appear on the website, as well as the colour, size and position of navigation menus, sidebars, logo, and all elements that make up the website’s front end, among others.
Themes are made by WordPress developers and designers. They are mostly written in PHP, with HTML, CSS and JavaScript, and shipped as a zip file, or hosted on directories such as the official WordPress themes directory.
There are several free and paid themes in the official WordPress theme directory. Some themes may be shipped as zip files that can be downloaded elsewhere online, or delivered directly to user, as may be the case for custom themes.
Child themes vs. Parent themes
A child theme is a theme that inherits most, if not all, of its styles and functionality from a Parent Theme (or Framework). Parent themes are primarily developed to allow child themes to inherit their features.
Child themes are used to override features of the parent theme to the extent as needed to make the website perform its unique functions, thus allowing different websites to perform and look unique, even though they may be built on the same framework.
Installing WordPress themes
WordPress themes can be installed from the Add Themes screen of the administration area, under the Themes sub-menu of the Appearance menu on single WordPress installs.
On multisite WordPress installs, the Add Themes screen is located under the Add New sub-menu of the Themes menu in the network administration area.
To install themes, a user must have the install_themes capability. By default, all users with Administrator role on single WordPress installs, or Super Administrator role on multisite installs, have this capability.
From the Themes screen, click on Add New or Add New Theme button.
The buttons leads to the Add Themes screen. You may type a theme name in the theme search form to preview or install a theme from thousands of free themes in the WordPress themes directory.
Click on any of the themes to preview, or hover over your preferred theme and click the Install button, to install the theme.
Alternatively, you may click the Upload Theme button on the Add Themes screen if you downloaded the theme in a zip format.
This would open a file upload form where you may select the file from your computer to upload. Click the Install Now button, to upload and install theme.
Activating installed themes
After installing the theme, click on the Activate link from the Installing Theme screen, or return to the Themes screen which lists all installed themes. You may then hover over a theme and click Activate, to activate the theme.
If using a child theme, be sure you have already installed the parent theme together with the child theme. Activate only the child theme.
The child theme would automatically inherit the parent theme’s features. Any functionality defined in the child theme would override any such similar functionality in the parent theme.
Conclusion
Themes are a powerful feature of WordPress, allowing to present content in creative ways. Care must be taken in choosing WordPress themes. Choose themes only from trusted sources, such as the official WordPress theme directory.
Bad actors are known to insert malicious code into themes and distribute them freely in theme market places. The official WordPress theme directory has people that manually review submitted themes before publicly listing them in the directory.
Avoid themes that come with features that lock you in. Features such as shortcodes and custom post types are plugin territory. Adding them to themes makes it impossible to switch themes without losing valuable functionality.