A WordPress theme controls how your site looks, and it also controls how fast it loads, whether it works on a phone, and how much trouble you will have in two years when you want to change something. Installing one takes a minute. Choosing one deserves longer, because a theme is the hardest thing on a WordPress site to replace after content has been built around it.
This covers what to look for, how to install a theme three ways, and the child theme step that stops your customisations being erased by the next update.
What actually matters when choosing
Is it maintained. Check the last update date. A theme untouched for a year will not be patched when a vulnerability is found, and will not be fixed when a WordPress release changes something underneath it. This outranks appearance.
What it weighs. Multipurpose themes that ship with sliders, portfolios, page builders and a dozen demo layouts carry all of that code whether you use it or not. A theme built for one job is usually faster than a theme that can do forty. If you are choosing between two you like equally, pick the smaller one.
Whether it locks you in. Themes that create content using their own shortcodes leave that content unreadable when you switch away, pages full of bracketed codes instead of text. Themes that build on standard WordPress blocks do not have this problem. This is the decision you will thank or curse yourself for later.
Whether it works on a phone. Most traffic is mobile. Open the theme's demo on your own phone rather than trusting the word responsive.
Support and documentation. For a free theme, look at whether recent forum threads get answers. For a commercial theme, that is what you are paying for.
Installing from the theme directory
Open Appearance then Themes then Add New. Themes here are hosted on wordpress.org, freely licensed and reviewed for basic standards, which means they meet a baseline, not that they are well built.
Use the feature filter to narrow by layout and features, then preview before installing. Click Install, then Activate.
Activating changes the live site immediately. On a site with visitors, use the Live Preview to look first, or try the theme on a staging copy.
Installing a theme you have as a file
Commercial themes arrive as a ZIP. Go to Appearance then Themes then Add New then Upload Theme, select the file and install.
Marketplace downloads often include documentation, licence files and the theme itself in one bundle. WordPress needs the theme ZIP specifically, uploading the whole bundle produces a "stylesheet is missing" error, which means you gave it the wrong file rather than that the theme is broken.
Only upload themes from the developer you bought from. Redistributed copies of commercial themes very frequently contain injected code, and the injection is the reason the copy exists.
Installing over SFTP
If the ZIP is larger than the upload limit, extract the theme folder into wp-content/themes using File Manager or an SFTP client. It appears in the theme list on the next page load.
Keep the structure intact: the theme's folder goes directly inside themes, containing style.css at its top level, not wrapped in another folder created by extraction.
Use a child theme before you customise anything
Edit a theme's files directly and the next update overwrites your work. Not might, will, because updating replaces the theme's files with the new version. A child theme is a small separate theme that inherits everything from the parent and holds only your changes, so updates leave your work alone.
Create a folder in wp-content/themes, for example parenttheme-child, containing a style.css that starts with:
/* Theme Name: Parent Theme Child Template: parenttheme */
The Template value must match the parent's folder name exactly, including case. Then add a functions.php in the same folder to load the parent's stylesheet:
<?php
add_action( 'wp_enqueue_scripts', function () {
wp_enqueue_style( 'parent-style',
get_template_directory_uri() . '/style.css' );
} );
Activate the child theme. The site looks identical, and your CSS and function changes now live somewhere updates cannot reach.
If your changes are only CSS, the Additional CSS box under Appearance then Customize is simpler and survives updates too. A child theme is for when you need to override templates or add functions.
Switching themes on a site that already has content
Switching is not just a visual change. Widgets get unassigned when the new theme has different sidebar areas. Menus need reassigning to the new theme's locations. Anything built with the old theme's shortcodes stops rendering.
Do it on a staging copy first if the site matters. If you must do it live, take a backup, switch, then work through menus, widgets and the homepage layout immediately instead of leaving the site half-configured.
Check the pages you did not build recently. The homepage gets attention; an old landing page full of the previous theme's shortcodes usually does not, until someone finds it.
Housekeeping
Keep the active theme, keep one default WordPress theme as a fallback for troubleshooting, and delete the rest. An inactive theme still has its code on disk, still needs patching, and is one more thing to update.
Turn on automatic updates for the themes you keep: see enabling automatic updates.
If a theme change makes the site slow, the theme is often not the whole story. Optimizing WordPress performance goes over how to find out what is actually costing the time before you blame the design.
Whether you also add a page builder is a separate decision with a longer tail. Page Builders Versus the Block Editor goes into the trade.