Overview

add_theme_support() tells WordPress which optional features your theme is ready to handle — such as letting WordPress manage the page title, enabling featured images, or letting the site owner upload a logo.

Features are opt-in so that old themes keep working. Each call is a promise: "my templates will display this properly."

The complete GDC Starter setup

<?php
function gdc_starter_setup() {
    // WordPress prints a correct <title> inside wp_head().
    add_theme_support( 'title-tag' );

    // Adds the Featured Image box to posts and pages.
    add_theme_support( 'post-thumbnails' );

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

    // Modern, valid markup for these core features.
    add_theme_support(
        'html5',
        array( 'search-form', 'comment-form', 'comment-list', 'gallery', 'caption', 'style', 'script', 'navigation-widgets' )
    );

    // A logo the site owner uploads in Appearance → Customize.
    add_theme_support(
        'custom-logo',
        array(
            'height'      => 80,
            'width'       => 240,
            'flex-height' => true,
            'flex-width'  => true,
        )
    );

    // Block editor features.
    add_theme_support( 'wp-block-styles' );
    add_theme_support( 'align-wide' );
    add_theme_support( 'responsive-embeds' );
    add_theme_support( 'editor-styles' );
    add_editor_style( 'assets/css/editor.css' );
}
add_action( 'after_setup_theme', 'gdc_starter_setup' );

What each feature gives you

Printing the custom logo

<div class="site-branding">
    <?php if ( has_custom_logo() ) : ?>
        <?php the_custom_logo(); ?>
    <?php else : ?>
        <a href="<?php echo esc_url( home_url( '/' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a>
    <?php endif; ?>
</div>

the_custom_logo() outputs a linked image with the correct alt text and srcset. The fallback to the site name means the header never ends up empty.

Making the editor match the front end

With editor-styles on, assets/css/editor.css is loaded into the block editor and automatically scoped so it only styles the content area:

/* assets/css/editor.css */
body {
    font-family: "Inter", system-ui, sans-serif;
    font-size: 18px;
    line-height: 1.7;
    color: #1e293b;
}

h1, h2, h3 {
    font-family: "Syne", system-ui, sans-serif;
    line-height: 1.2;
}

a {
    color: #6c63ff;
}

Tip: When the editor looks like the published page, clients make far fewer "it looked different when I wrote it" support requests.

Checking for a feature

current_theme_supports( 'post-thumbnails' ) returns whether a feature is on. Plugins use it to adapt to your theme — another reason to declare features honestly.

Practice

• Add every feature above, then remove any <title> from header.php and confirm the browser tab still shows the right title.
• Upload a logo in Appearance → Customize → Site Identity and confirm it replaces the site name.
• Write editor.css and compare a post in the editor and on the front end.