Conditional logic on code injection

Check WordPress version the site is using to decide what action hook to use

function custom_code() {
    ?>
    <!-- some code -->
    <?php
}

if ( did_action( 'wp_body_open' ) ) {
    add_action( 'wp_body_open', 'custom_code' );
} else {
    add_action( 'custom_theme_hook', 'custom_code' );
}