functions.php

<?php
function my_custom_pre_main() {
    echo 'This content will be visible after the content div and before the main content.';
}

function only_in_single_posts() {
    if ( is_single() ) {
        add_action( 'shoestrap_pre_main', 'my_custom_pre_main' );
    }
}
add_action( 'init', 'only_in_single_posts' );