Return the_content() with custom div class for a subset of posts

This can be done in multiple different ways. For example:

1. Using the_content filter hook:

Use the_content filter hook to wrap the output of the_content() function call with your required div.

You may try the following CODE in your theme’s functions.php file or in a custom plugin:

add_filter( 'the_content', 'wpse_specific_the_content' );
function wpse_specific_the_content( $content ) {
    // logic to identify unique posts
    // for example, using specific post ID
    $unique_ids = array (12, 14, 21);
    // current post ID
    $id = get_the_id();
    if( in_array( $id, $unique_ids ) ) {
        return '<div class="unique-posts">' . $content . '</div>';
    }
    return $content;
}

2. Use post_class filter hook:

You don’t actually need to wrap the_content() in additional div. Instead, you may use the post_class filter hook.

This way, you will be able to assign unique CSS classes to different posts and pages that needs them and style them based on those CSS classes.

For example:

add_filter( 'post_class', 'wpse_filter_post_class' );
function wpse_filter_post_class( $class ) {
    $unique_ids = array (12, 14, 21);
    $id = get_the_id();
    if( in_array( $id, $unique_ids ) ) {
        // add these custom CSS classes to matching posts
        $add = array( 'custom-posts', 'some-class' );
        $class = array_merge( $add, $class );
    }
    return $class;
}

Now target this class the way you need:

.custom-posts {
    background-color: gold;
}
.custom-posts .entry-content {
    background-color: red;
}

3. Use body_class filter hook:

If you need more generic control over the entire page design (compared to what post_class will provide), then you may modify <body> class using the body_class filter hook, the same post_class filter hook is show above.

Like this:

add_filter( 'body_class', 'wpse_filter_body_class' );
function wpse_filter_body_class( $class ) {
    // this check is needed for body_class hook,
    // because the current page may not be a single post at all.
    if ( is_single() ) {
        $unique_ids = array (12, 14, 21);
        $id = get_the_id();
        if( in_array( $id, $unique_ids ) ) {
            $add = array( 'custom-body' );
            $class = array_merge( $add, $class );
        }   
    }
    return $class;
}

Now you can use the following CSS to target custom post page:

body.custom-body .any-inner-content-class {
    color: blue;
}

Since this custom class is added to the <body> tag, this way you’ll be able to target any element within the page.