Themes

Themes control the visual appearance and layout of a WordPress website. A theme is a collection of files (PHP, CSS, JavaScript, and template files) that determine how your content is displayed to visitors. Modern WordPress supports both classic PHP-based themes and block themes designed for Full Site Editing.

Category: Cms (also: Design)

Author: Atomic Glue Editorial Team

## Definition

A theme is a collection of files that determines the visual design and layout of a [WordPress](/glossary/wordpress) website. At minimum, a theme requires two files: `style.css` (which includes the theme header with metadata) and `index.php` (the fallback template). Full themes include multiple template files following the [Template Hierarchy](/glossary/template-hierarchy), along with CSS, JavaScript, images, and sometimes PHP functions. WordPress themes can be categorized into two architectural types: **classic themes** (using PHP template files like `header.php`, `footer.php`, `single.php`) and **block themes** (using HTML template files with block markup, designed for [Full Site Editing (FSE)](/glossary/full-site-editing-fse)). Block themes represent the future of WordPress theming, allowing complete site customization through the Site Editor without touching code.

## Classic Themes vs Block Themes

**Classic themes** use PHP template files to render pages. The theme's `functions.php` file registers features, enqueues scripts and styles, and defines custom functionality. Content is edited in the Classic Editor or Gutenberg, but the overall page structure (header, footer, sidebar) is defined by PHP templates. Customizing a classic theme beyond its built-in options requires editing PHP files or a child theme. **Block themes (FSE themes)** use HTML files with block markup. Instead of PHP template files like `header.php`, block themes use `templates/index.html` and `parts/header.html` with Gutenberg blocks. The entire site (header, footer, templates, styles) can be customized through the WordPress Site Editor without any PHP knowledge. Block themes are the recommended approach for new WordPress projects as of WordPress 6.x. They're cleaner, more secure (less PHP = fewer vulnerabilities), and give content editors more control over site-wide design. The transition is gradual, and many production sites still use classic themes. Both work fine. Block themes are the future.

## Parent Themes and Child Themes

A **child theme** inherits the functionality and styling of another theme (the parent theme) while allowing you to make modifications without altering the parent theme's files. This is important because WordPress theme updates overwrite theme files. If you modify a parent theme directly, your changes are lost on update. Child themes work by: 1. Creating a new directory in `/wp-content/themes/` 2. Adding a `style.css` with a Template header pointing to the parent theme directory 3. Optionally adding `functions.php` to enqueue parent styles and add custom functions 4. Overriding any parent template file by creating a file with the same name Child themes are considered a best practice for any WordPress site that needs customizations beyond theme settings. They protect your changes during updates and keep your customizations isolated and maintainable.

## Choosing a Theme

The right theme depends on your site's purpose and your team's capabilities: **For content-driven sites:** Choose a lightweight, well-coded block theme. GeneratePress, Kadence, and Twenty Twenty-Five are excellent starting points. Fast loading, accessibility-ready, FSE-compatible. **For ecommerce:** Choose a WooCommerce-compatible theme. Storefront (official WooCommerce theme), Astra, or Blocksy. Performance matters more than flashy features for stores. **For design-forward sites:** Consider a premium theme from ThemeForest or a [Webflow](/glossary/webflow) build if WordPress theming is too limiting. Sometimes the right tool isn't a WordPress theme at all. **For custom projects:** A custom block theme built from scratch. This is the best approach for serious sites. No bloat, no unnecessary features, complete design control. Atomic Glue builds custom themes for clients who need exactly what they want, not what a theme author decided. **Red flags when choosing themes:** Auto-updating sliders, bundled page builders, 50+ demo sites to import, 'premium' themes from untrusted sources. These are performance and security liabilities.

## In code

// Minimum child theme style.css header
/*
Theme Name: Atomic Glue Child
Template: generatepress
*/

// functions.php to enqueue parent styles
add_action( 'wp_enqueue_scripts', 'ag_child_enqueue_styles' );
function ag_child_enqueue_styles() {
    wp_enqueue_style( 
        'parent-style', 
        get_template_directory_uri() . '/style.css' 
    );
}

## Common questions

Q: What's the difference between a theme and a page builder?

A: A theme controls your site's overall design and layout. A page builder (like Elementor or Beaver Builder) is a plugin that adds drag-and-drop editing within pages. You can use a theme with or without a page builder.

Q: How often should I change my theme?

A: Rarely. Changing themes is a significant event that affects design, layout, and potentially functionality. Choose carefully the first time, customize with a child theme, and only switch when truly necessary.

Q: Are free WordPress themes safe?

A: Free themes from the WordPress.org directory are reviewed and generally safe. Free themes from third-party sites are risky. Premium themes are not inherently safer than free ones.

## Key takeaways

• Themes control the visual design of a WordPress site through template files, CSS, and JavaScript.
• Block themes (FSE-compatible) are replacing classic PHP-based themes as the modern standard.
• Always use child themes for customizations to protect changes during parent theme updates.
• Choose lightweight, well-coded themes. Avoid bloated multipurpose themes with unnecessary features.

## Related entries

• [WordPress](atomicglue.co/glossary/wordpress)
• [Plugins](atomicglue.co/glossary/plugins)
• [Template Hierarchy](atomicglue.co/glossary/template-hierarchy)
• [Full Site Editing (FSE)](atomicglue.co/glossary/full-site-editing-fse)
• [Page Builder](atomicglue.co/glossary/page-builder)

Last updated July 2025. Permalink: atomicglue.co/glossary/themes