Automatic value for custom fields for posts

Try meta_query to get posts having theme_settings_post_show == true (for new posts) and theme_settings_post_show not set at all (for old posts):

<?php
$queryArguments = array(
    'posts_per_page' => -1,
    'post_type'     => 'post',
    'meta_query' => array(
       'relation' => 'OR', // value is not set or true
        array(
         'key' => 'theme_settings_post_show',
         'value' => '', // can be any value, since it does not exists
         'compare' => 'NOT EXISTS',
        ),
        array(
         'key' => 'theme_settings_post_show',
         'value' => true,
         'compare' => '=',
        ),
    ),
);