A WordPress theme is the package of files that decides how your site looks: its layouts, colors, typography, header, footer and the templates used for posts, pages and archives. Your content stays in the database; the theme is the presentation layer wrapped around it, which is why you can switch themes without losing a single post. This guide explains what is inside a theme, how block themes differ from classic ones, how to install and customize one, and what to check before you pay for a premium design.
What a WordPress theme actually does
Think of WordPress as three layers: core (the engine), plugins (features) and the theme (design). Exactly one theme is active at a time. When a visitor opens a URL, WordPress works out what kind of page it is (a single post, a category archive, the home page, a 404) and asks the active theme for the matching template. The theme supplies the structure and styling; WordPress fills it with content.
A good rule of thumb: a theme should handle presentation, and plugins should handle functionality. If a theme registers your custom post types, contact forms or SEO settings, that content and behavior can vanish when you switch designs. Well-built themes keep to their lane, which is one of the review guidelines for themes listed in the official wordpress.org directory.
What is inside a theme folder
Every theme lives in its own folder under wp-content/themes/. The minimum requirements differ by theme type, but the common files look like this:
| File or folder | Purpose | Theme type |
|---|---|---|
style.css | Header comment with the theme name, version, author and license; may also hold CSS | Both (required) |
theme.json | Global settings and styles: color palette, font sizes, spacing, layout widths, per-block styles | Both (essential for block themes) |
templates/ | HTML templates made of block markup, e.g. index.html, single.html | Block (templates/index.html required) |
parts/ | Reusable template parts such as header and footer | Block |
patterns/ | Pre-designed block layouts users can insert | Block (also usable in classic) |
index.php, single.php, header.php… | PHP templates following the template hierarchy | Classic (index.php required) |
functions.php | Registers features, enqueues scripts and styles, adds hooks | Both (optional) |
screenshot.png | Thumbnail shown under Appearance → Themes | Both |
The official Theme Handbook documents each of these in detail, including the template hierarchy that decides which file is used for which URL.
Block themes vs classic themes
This is the biggest distinction in the theme world today, and it affects how you will edit your site day to day.
Classic themes
Classic themes build pages with PHP template files. You customize them through the Customizer (Appearance → Customize), widget areas (Appearance → Widgets) and menus (Appearance → Menus), plus whatever options panel the theme author added. Headers and footers are generally fixed in code unless the theme exposes settings for them. Many excellent, mature themes are classic, and they remain fully supported by WordPress.
Block themes
Block themes build every part of the site, including headers, footers and archive layouts, out of blocks. You edit them visually in the Site Editor (Appearance → Editor), and global design choices live in theme.json and the Styles panel. There is no separate Customizer or widget screen by default, because everything is a block. Block themes arrived with Full Site Editing in WordPress 5.9 and have matured with every release since.
| Classic theme | Block theme | |
|---|---|---|
| Templates | PHP files | HTML files of block markup |
| Customizing | Customizer, widgets, theme options | Site Editor and Styles |
| Header and footer | Usually coded | Editable template parts |
| Global styles | CSS, sometimes theme.json | theme.json plus the Styles panel |
| Best for | Existing sites, heavy PHP customization, page-builder workflows | New sites, visual editing, fewer plugins |
Neither type is automatically better. If your site relies on a page builder like Elementor, a lightweight classic theme designed for that builder is a sensible choice. If you want to edit everything with native WordPress tools, a block theme is the more future-facing option. Our block themes guide and theme.json guide go deeper on how they are built.
How to install and switch a WordPress theme
From the free directory
- Go to Appearance → Themes and click Add New Theme.
- Search or filter the wordpress.org theme directory listing. Use the “Block Themes” filter if you want Site Editor support.
- Click Preview to see it with sample content, then Install, then Activate.
Uploading a premium theme
- Download the theme zip from the seller. Some vendors ship a bundle containing documentation and demo files; upload only the installable theme zip, which contains
style.cssat its top level. - Go to Appearance → Themes → Add New Theme → Upload Theme, choose the zip and click Install Now.
- Activate it, then follow the vendor’s setup steps (required plugins, demo import, license key for updates).
Switching safely on a live site
- Take a full backup (files and database) first.
- Test the new theme on a staging copy, or use Live Preview from the Themes screen.
- Check menus, widgets, the home page setting (Settings → Reading) and featured image sizes afterwards. Moving between classic and block themes in particular means rebuilding navigation and widget content as blocks.
- Watch for theme-specific shortcodes left behind in old posts; they will show as raw text once the theme that defined them is gone.
Customizing a theme without losing your changes
The most important rule: do not edit the theme’s own files directly. The next theme update will overwrite them. Instead, use one of these layers, roughly in order of preference:
- Built-in settings. In a block theme, the Site Editor and Styles panel store your changes in the database, separate from the theme files. Our Site Editor tutorial covers this step by step. In a classic theme, use the Customizer and theme options.
- Block-level styling. Core blocks expose color, typography, spacing and border controls. Plugins can add more; our free StepFox Looks plugin, for example, adds responsive per-block styles without writing CSS.
- Additional CSS. For small tweaks, use Appearance → Customize → Additional CSS (classic) or the Additional CSS option in the Styles panel (block themes).
- A child theme. For template or PHP changes, create a child theme that inherits from the parent and overrides only what you need. See our child themes guide.
One caveat for block themes: once you edit a template in the Site Editor, your copy is saved in the database and takes precedence over the file in the theme. Future theme updates to that template will not appear until you reset it (the template’s menu offers a reset or “clear customizations” option).
Free vs premium themes
Themes in the wordpress.org directory are free, GPL-licensed and pass a manual review covering security, coding standards and licensing. The default Twenty-something themes, built by the WordPress project itself, are a reliable baseline. Read more in our guide to free WordPress themes.
Premium themes are sold by independent developers and shops. What you typically pay for is not the code license (most are GPL too) but the extras: specialized designs, demo content, direct support and a period of updates. Pricing structures vary: one-off purchases, annual licenses that include updates and support, or memberships covering many themes. Specialist niches such as newspapers, magazines and sports leagues are where premium themes most often add real value, because those layouts are hard to assemble from a general-purpose theme.
Avoid “nulled” themes, which are pirated copies of premium themes shared for free. They frequently contain injected malware or backdoors and never receive updates.
What to check before buying a WordPress theme
- Type. Block theme, classic theme, or built for a specific page builder? Make sure it matches how you want to edit.
- Live demo. Test it on your phone, check the article page and archive pages, not just the home page.
- Speed. Run the demo through PageSpeed Insights. A demo is a best case; if it is slow there, it will be slow for you.
- Update history. Look for a changelog with recent releases that track current WordPress versions.
- Support terms. How long does support last, and what does it cover? How are updates delivered?
- Lock-in. Does it bundle proprietary page builders or shortcodes that will leave a mess if you switch later?
- Accessibility and translation. Keyboard navigation, readable contrast and translation-ready strings matter for many sites.
- Refund policy. Know what happens if it does not fit.
If you run a news, magazine or sports site, you can browse block-based designs for those niches in our theme catalogue and apply the same checklist.
Frequently asked questions
Will changing my WordPress theme delete my content?
No. Posts, pages, media and users live in the database. What can change is anything the old theme provided: widget placement, menu locations, theme-specific shortcodes and custom post types registered by the theme.
Can I use two themes at once?
Only one theme is active per site. A child theme plus its parent is the closest thing, and on a multisite network each site can run a different theme.
Do block themes work with page builders?
Often, yes, but you end up with two editing systems. If you plan to build everything in a page builder, a theme designed for that builder is usually simpler.
Key takeaways
- A WordPress theme controls presentation; your content stays in the database when you switch.
- Block themes use HTML templates,
theme.jsonand the Site Editor; classic themes use PHP templates and the Customizer. - Install from Appearance → Themes → Add New Theme, or upload a premium zip, and always back up before switching.
- Customize through settings, block styles, Additional CSS or a child theme, never by editing the theme’s files.
- Before buying, check the demo, speed, update history, support terms and lock-in risk.
