Enqueue a theme style and opt into WP's small-CSS inlining.

functions.php

<?php

/**
 * Enqueue a theme style and opt into WP's small-CSS inlining.
 *
 * If the file is small enough (per core’s internal threshold), WP will inline it.
 * Otherwise it remains a normal <link>.
 *
 * @param string      $handle         Unique style handle.
 * @param string      $relative_path  Path relative to the theme root (e.g. 'assets/css/theme.css').
 * @param array       $deps           Optional. Style dependencies.
 * @param string|bool $ver            Optional. Version string. Null/empty uses theme version. False omits version.
 * @param string      $media          Optional. Media attribute. Default 'all'.
 */
function enqueue_smart_style(
    string $handle,
    string $relative_path,
    array $deps = [],
    $ver = null,
    string $media = 'all'
) {
    $abs = get_theme_file_path( $relative_path );
    if ( ! $abs || ! file_exists( $abs ) ) {
        return;
    }

    $uri = get_theme_file_uri( $relative_path );

    // Versioning: default to theme version unless explicitly disabled with false.
    if ( $ver === null || $ver === '' ) {
        $ver = wp_get_theme()->get( 'Version' );
    } elseif ( $ver === false ) {
        $ver = null; // omit ?ver=
    }

    // Register as usual.
    wp_register_style( $handle, $uri, $deps, $ver, $media );

    // Opt in to core's small-CSS inlining: must be set before enqueue.
    // Core will read the file at this absolute path and inline if it's small.
    wp_style_add_data( $handle, 'path', $abs );

    wp_enqueue_style( $handle );
}

function enqueue_scripts() {
	enqueue_smart_style( 'my-theme','style.css' );
}

add_action( "wp_enqueue_scripts", "enqueue_scripts", 100 );

CSS inlining limit was doubled from 20k to 40k in WP 6.9, https://core.trac.wordpress.org/ticket/63018 Thanks Adam.