Highlight posts that belong to current sub category

Instead of:

$categories = get_the_category(); 
$categories_id = $categories[0]->cat_ID;    

…use:

$categories = wp_list_pluck( get_the_category(), 'term_id' );

Now you can correctly use in_array to search all the post’s categories, rather than just the first:

if ( in_array( $thisTrueCat->term_id, $categories ) )  { ... }