Wp Theme Standards

WordPress Legacy Theme Standards

This skill defines the mandatory standards for building WordPress themes using the legacy (classic) theme architecture. No block themes, no Full Site Editing (FSE), no theme.json.

Required Theme Files

Every theme MUST include these files:

style.css Headers

The style.css file MUST begin with the theme declaration comment. This is how WordPress identifies the theme.

/*
Theme Name:   Starter Theme
Theme URI:    https://example.com
Author:       Developer Name
Author URI:   https://example.com
Description:  A custom legacy WordPress theme.
Version:      1.0.0
License:      GNU General Public License v2 or later
License URI:  https://www.gnu.org/licenses/gpl-2.0.html
Text Domain:  starter
*/

Note: Do not put actual styles in style.css. Use it only for the header declaration. All styles go in assets/css/styles.css (or similar), enqueued via functions.php.

Theme Directory Structure

theme-name/
├── assets/
│   ├── css/
│   │   └── styles.css          # Main design system stylesheet
│   ├── js/
│   │   └── main.js             # Main client-side JS
│   └── images/                 # Theme images (logo fallback, icons, etc.)
├── inc/
│   ├── theme-setup.php         # Theme supports, nav menus, content width
│   ├── i18n.php                # Internationalization helpers (if bilingual)
│   ├── performance.php         # WebP delivery + prefix_image() right-size helper
│   └── scf-fields.php          # SCF/ACF custom field definitions
├── template-parts/
│   ├── section-hero.php        # Reusable section templates
│   ├── section-services.php
│   └── ...
├── functions.php               # Main functions file (requires inc/ files)
├── header.php                  # Site header
├── footer.php                  # Site footer
├── front-page.php              # Homepage template
├── index.php                   # Fallback template
├── style.css                   # Theme declaration (headers only)
└── screenshot.png              # Theme preview image

Asset Enqueueing

NEVER add <link> or <script> tags directly in templates. Always use WordPress enqueueing functions.

Styles

function prefix_scripts() {
    // Google Fonts (external, no version needed)
    wp_enqueue_style(
        'prefix-fonts',
        'https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap',
        array(),
        null
    );

    // Main stylesheet with filemtime() cache busting
    wp_enqueue_style(
        'prefix-style',
        get_template_directory_uri() . '/assets/css/styles.css',
        array('prefix-fonts'),
        filemtime(get_template_directory() . '/assets/css/styles.css')
    );
}
add_action('wp_enqueue_scripts', 'prefix_scripts');

Scripts

// Main JavaScript — loaded in footer (last param = true)
wp_enqueue_script(
    'prefix-main',
    get_template_directory_uri() . '/assets/js/main.js',
    array(),
    filemtime(get_template_directory() . '/assets/js/main.js'),
    true
);

Cache Busting

Always use filemtime() for the version parameter on local assets. This forces browsers to re-download the file whenever it changes, without manual version bumping.

filemtime(get_template_directory() . '/assets/css/styles.css')

For external assets (CDN fonts, libraries), pass null as the version to omit the query string.

wp_localize_script() for Passing PHP Data to JavaScript

Use wp_localize_script() to safely pass PHP data (dynamic content, URLs, translations) to JavaScript.

// Pass calculator data to JS on the pricing page
if (is_page('pricing')) {
    $calculator_data = prefix_get_calculator_data();
    wp_localize_script('prefix-main', 'prefixCalculator', $calculator_data);
}

// Pass i18n data to JS for all pages
wp_localize_script('prefix-main', 'prefixI18n', array(
    'currentLang' => prefix_get_current_lang(),
    'strings'     => prefix_get_js_translations(),
));

In JavaScript, access the data via the global variable name:

console.log(prefixCalculator.setupOptions);
console.log(prefixI18n.currentLang); // 'en' or 'es'

Page-Specific Asset Enqueueing

Load page-specific CSS and JS only when needed using is_page_template() or is_page().

function prefix_scripts() {
    // ... main styles/scripts ...

    // Software page: dedicated CSS + JS
    if (is_page_template('page-software.php')) {
        wp_enqueue_style(
            'prefix-software-style',
            get_template_directory_uri() . '/assets/css/software.css',
            array('prefix-style'),
            filemtime(get_template_directory() . '/assets/css/software.css')
        );
        wp_enqueue_script(
            'prefix-software-js',
            get_template_directory_uri() . '/assets/js/software.js',
            array('prefix-main'),
            filemtime(get_template_directory() . '/assets/js/software.js'),
            true
        );
    }
}
add_action('wp_enqueue_scripts', 'prefix_scripts');

Theme Supports

Register all required theme features inside an after_setup_theme hook.

function prefix_setup() {
    // Let WordPress manage the document <title>
    add_theme_support('title-tag');

    // Enable featured images
    add_theme_support('post-thumbnails');

    // Custom logo support
    add_theme_support('custom-logo', array(
        'height'      => 100,
        'width'       => 340,
        'flex-height' => true,
        'flex-width'  => true,
    ));

    // HTML5 markup for core elements
    add_theme_support('html5', array(
        'search-form',
        'comment-form',
        'comment-list',
        'gallery',
        'caption',
        'style',
        'script',
    ));

    // RSS feed links in <head>
    add_theme_support('automatic-feed-links');

    // Register navigation menus
    register_nav_menus(array(
        'primary' => __('Primary Navigation', 'theme-slug'),
        'footer'  => __('Footer Navigation', 'theme-slug'),
    ));
}
add_action('after_setup_theme', 'prefix_setup');

Content Width

Set the global content width for embeds and images.

function prefix_content_width() {
    $GLOBALS['content_width'] = apply_filters('prefix_content_width', 1280);
}
add_action('after_setup_theme', 'prefix_content_width', 0);

Security: Output Escaping

Every dynamic value rendered in HTML MUST be escaped. No exceptions.

Rules

• Plain text in tags: esc_html()
• URLs anywhere: esc_url()
• Attribute values (class, id, data-*): esc_attr()
• Rich HTML content from WYSIWYG/editor fields: wp_kses_post()
• Never output raw get_field(), $_GET, $_POST, or any user input without escaping

Security: Input Sanitization

Sanitize all input before saving to the database.

// Example: sanitize URL parameter
if (isset($_GET['lang'])) {
    $lang = sanitize_text_field($_GET['lang']);
}

WordPress Hooks Reference

The hooks below are the ones most commonly used in theme development, listed in the order they typically fire.

Querying Posts

NEVER use query_posts(). It modifies the main query and causes bugs.

ALWAYS use WP_Query for custom queries.

$args = array(
    'post_type'      => 'post',
    'posts_per_page' => 6,
    'orderby'        => 'date',
    'order'          => 'DESC',
);
$query = new WP_Query($args);

if ($query->have_posts()) :
    while ($query->have_posts()) : $query->the_post();
        get_template_part('template-parts/content', get_post_type());
    endwhile;
    wp_reset_postdata();
endif;

Always call wp_reset_postdata() after a custom WP_Query loop.

No Inline Styles or Scripts

• Never add <style> blocks in PHP templates
• Never add <script> blocks in PHP templates
• Never use inline style="" attributes on elements

All CSS goes in .css files. All JS goes in .js files. Both are enqueued via wp_enqueue_scripts.

The only exceptions are:

• wp_head actions for <meta> tags, <link rel="preload">, and <script type="application/ld+json"> (schema markup)
• SVG admin display fix in admin_head (minimal inline style)

Performance Optimizations

Font Preconnect

Add preconnect hints for external font providers to speed up loading.

function prefix_add_preconnect() {
    echo '<link rel="preconnect" href="https://fonts.googleapis.com">' . "\n";
    echo '<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>' . "\n";
}
add_action('wp_head', 'prefix_add_preconnect', 1);

LCP Image Preloading

Preload the Largest Contentful Paint (LCP) element (usually the hero image) on the homepage.

function prefix_preload_lcp_image() {
    if (is_front_page()) {
        $hero_image = prefix_get_field('hero_image');
        $hero_image_url = $hero_image ? $hero_image['url'] : prefix_asset('images/hero-image.png');
        echo '<link rel="preload" as="image" href="' . esc_url($hero_image_url) . '" fetchpriority="high">' . "\n";
    }
}
add_action('wp_head', 'prefix_preload_lcp_image', 2);

The preloaded LCP <img> itself must carry fetchpriority="high" + width/height. If the hero is a background video, keep the poster <img> as the LCP element and lazy-load the <video> via JS on desktop only (never mobile / navigator.connection.saveData).

Image Delivery — WebP + right-sizing (inc/performance.php)

Ship inc/performance.php (in the starter) in every theme. It handles the image-delivery waste Lighthouse flags, which image_editor_output_format alone does not cover:

1. image_editor_output_format → WebP for new attachment sub-sizes.
2. wp_generate_attachment_metadata → also writes a WebP sibling for the full-size original (so raw-URL / CSS-background usage benefits).
3. A template_redirect output-buffer that rewrites any uploads/*.jpg|png with a .webp sibling → .webp in the finished HTML (covers src, srcset, and inline background-image — including SCF field URLs that bypass WP's attachment pipeline). Runs once per page-cache build.
4. prefix_image($field, $size, $attr) — templates use this instead of echo $field['url'] so images get a srcset sized to the slot. Always pass sizes matching the real display width (full-bleed 100vw, split (max-width: 899px) 100vw, 50vw, fixed logo 136px). Serving a 2200px original in a 400px slot is the top oversized-image finding.

For a theme seeded from an existing demo (images already uploaded), batch-generate the .webp siblings once so (3) picks them up:

find wp-content/uploads -type f \( -iname '*.jpg' -o -iname '*.png' \) \
  -exec sh -c 'f="$1"; w="${f%.*}.webp"; [ -f "$w" ] || magick "$f" -quality 82 "$w"' _ {} \;

Disable WordPress Emojis

Remove the emoji detection script and styles that WordPress loads on every page.

function prefix_disable_emojis() {
    remove_action('wp_head', 'print_emoji_detection_script', 7);
    remove_action('admin_print_scripts', 'print_emoji_detection_script');
    remove_action('wp_print_styles', 'print_emoji_styles');
    remove_action('admin_print_styles', 'print_emoji_styles');
    remove_filter('the_content_feed', 'wp_staticize_emoji');
    remove_filter('comment_text_rss', 'wp_staticize_emoji');
    remove_filter('wp_mail', 'wp_staticize_emoji_for_email');
}
add_action('init', 'prefix_disable_emojis');

Hide WordPress Version

Remove the generator meta tag that exposes the WordPress version.

remove_action('wp_head', 'wp_generator');

SVG Upload Support

Allow SVG file uploads in the WordPress media library.

function prefix_allow_svg_upload($mimes) {
    $mimes['svg']  = 'image/svg+xml';
    $mimes['svgz'] = 'image/svg+xml';
    return $mimes;
}
add_filter('upload_mimes', 'prefix_allow_svg_upload');

function prefix_fix_svg_display() {
    echo '<style>
        .attachment-266x266, .thumbnail img {
            width: 100% !important;
            height: auto !important;
        }
    </style>';
}
add_action('admin_head', 'prefix_fix_svg_display');

function prefix_check_filetype($data, $file, $filename, $mimes) {
    $filetype = wp_check_filetype($filename, $mimes);
    return array(
        'ext'             => $filetype['ext'],
        'type'            => $filetype['type'],
        'proper_filename' => $data['proper_filename'],
    );
}
add_filter('wp_check_filetype_and_ext', 'prefix_check_filetype', 10, 4);

Custom Body Classes

Add contextual CSS classes to the <body> tag for page-specific styling.

function prefix_body_classes($classes) {
    if (is_front_page()) {
        $classes[] = 'home-page';
    }
    if (is_page('pricing')) {
        $classes[] = 'pricing-page';
    }
    if (is_page_template('page-software.php')) {
        $classes[] = 'software-page';
    }
    if (is_singular('post')) {
        $classes[] = 'single-post-page';
    }
    if (is_archive()) {
        $classes[] = 'archive-page';
    }
    return $classes;
}
add_filter('body_class', 'prefix_body_classes');

SCF/ACF as Required Dependency

All themes built with this system use Secure Custom Fields (SCF) or Advanced Custom Fields (ACF) as the custom fields plugin. SCF is an ACF-compatible fork and uses the same API (get_field(), the_field(), have_rows(), etc.).

Options Page Registration

Register an options page for site-wide settings (logo, footer content, social links, etc.).

function prefix_register_options_page() {
    if (function_exists('acf_add_options_page')) {
        acf_add_options_page(array(
            'page_title' => 'Site Settings',
            'menu_title' => 'Site Settings',
            'menu_slug'  => 'prefix-settings',
            'capability' => 'edit_posts',
            'redirect'   => false,
            'icon_url'   => 'dashicons-admin-generic',
            'position'   => 30,
        ));
    }
}
add_action('acf/init', 'prefix_register_options_page');

Retrieving Option Fields

// Options page fields use 'option' as the post ID
$logo = get_field('site_logo', 'option');
$footer_text = get_field('footer_copyright', 'option');

Helper Functions

Create small utility functions to keep templates clean.

/**
 * Get theme asset URL
 */
function prefix_asset($path) {
    return get_template_directory_uri() . '/assets/' . ltrim($path, '/');
}

/**
 * Get site logo with fallback
 */
function prefix_get_logo() {
    $logo = get_field('site_logo', 'option');
    if ($logo) {
        return $logo;
    }
    return prefix_asset('images/logo.svg');
}

Schema.org Structured Data

Add JSON-LD structured data for SEO and AI search optimization.

function prefix_output_schema_markup() {
    $site_url  = home_url();
    $site_name = get_bloginfo('name');

    $schema = array(
        '@context'    => 'https://schema.org',
        '@type'       => 'Organization',
        '@id'         => $site_url . '/#organization',
        'name'        => $site_name,
        'url'         => $site_url,
        'description' => 'A brief description of the business.',
    );

    echo '<script type="application/ld+json">' . "\n";
    echo wp_json_encode($schema, JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT);
    echo "\n" . '</script>' . "\n";
}
add_action('wp_head', 'prefix_output_schema_markup', 5);

SEO Meta Descriptions

Add meta description tags, but defer to SEO plugins if present.

function prefix_add_meta_description() {
    // Skip if Yoast or Rank Math is active
    if (defined('WPSEO_VERSION') || defined('RANK_MATH_VERSION')) {
        return;
    }

    $description = '';

    if (is_front_page()) {
        $description = 'Your site description here.';
    } elseif (is_singular('post')) {
        $post = get_post();
        $description = has_excerpt() ? get_the_excerpt($post) : wp_trim_words(strip_tags($post->post_content), 30, '...');
    }

    if ($description) {
        echo '<meta name="description" content="' . esc_attr($description) . '">' . "\n";
    }
}
add_action('wp_head', 'prefix_add_meta_description', 1);

Naming Conventions

• All PHP functions use a unique prefix: prefix_ (replace with your theme slug, e.g., kairo_, starter_)
• Template parts are named section-*.php for content sections, footer-*.php for footer variants
• Page templates are named page-*.php (e.g., page-pricing.php, page-software.php)
• CSS classes use BEM: .block__element--modifier
• JS files use lowercase with hyphens: main.js, software.js

Navigation class contract

The nav walker (generates markup) and the header CSS (styles it) MUST agree on one shared set of class names. Use exactly these — do not invent alternates:

States: .is-open (added to .nav__item--has-children when its submenu is expanded) and aria-expanded ("true"/"false" attribute on .nav__toggle, kept in sync with .is-open).

Baseline alignment rule (mandatory): toggle items must sit on the same text baseline as plain links — never let the presence of a dropdown arrow shift a label up/down relative to its siblings. Apply display:flex; align-items:center to both .nav__link and .nav__toggle.

.nav__link,
.nav__toggle {
    display: flex;
    align-items: center;
}

Summary Checklist

• style.css has required WordPress headers
• All assets enqueued via wp_enqueue_style() / wp_enqueue_script()
• filemtime() used for cache busting on all local assets
• All theme supports registered in after_setup_theme
• All dynamic output escaped with appropriate function
• No query_posts() anywhere
• No inline styles or scripts in templates
• Emojis disabled, WP version hidden
• SVG uploads enabled
• Custom body classes added
• SCF/ACF options page registered
• Schema.org structured data output
• Meta descriptions added with SEO plugin check
• Font preconnect and LCP preloading configured