post_class problem

hello, i am trying to target and edit the look of a specific posts, shown in the front page,based on their tag. I have added the post_class or

<?php
<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

in my child themes /css/index.php file. When i go to edit the desired post based on its tag, it does not work. my css code is

.tag-review .entry-title {
border:none;
}

can somebody help me solve this problem. In my child theme,I have a wp-content/theme folder/css/index.php, /images/index.php, and a /js/index.php. Which index.php does

the<?php
<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

go in? The “.review” class is the tag of the posts im trying to target and “.entry-title” is what i am trying to change in the post. Can some one guide me in the right direction please?…Thank you.

Hello alchymyth, thanks so much for the reply. I am using headway themes and i am running a wamp set up so i cant provide a live link. In my child theme i have three index.php files in different locations.
wp-content/themes/custom code/css/index.php,
wp-content/themes/custom code/images/index.php, and
wp-content/themes/custom code/js/index.php

I inserted

<?php
<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

inside all of them.

This is the html that was outputed

<div id="post-51" class="post-51 post type-post status-publish format-standard hentry category-pc tag-review author-test">

but the same html is there with or without the

<?php
<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

inside the index.php files. I am starting to think that i need an index.php file in my main child theme folder

wp-content/themes/custom code/index.php.

but when i tried to put an empty index.php file their my front page goes blank.

I have copied the parents theme index.php file over to the child theme. ( wp-content/themes/custom-code/index.php ). The blank page problem is solved. The site shows properly now. i have added the post_class to the child themes index.php file like this.

<?php

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

?>

When i load the site i get a parse error.
” Parse error: syntax error, unexpected ‘<‘ in C:\wamp\www\wordpress-test\wp-content\themes\custom-code\index.php on line 19“.

in my text editor I checked line 19 and it is this.

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

i copied it straight out of the post_class wordpress page.
codex.wordpress.org/Function_Reference/post_class

alchymyth, i removed the <?php and the ?> opening and closing tags. Now the site works and no parse errors. Now when i go to target all the post with the tag “review“, every post including the ones with a different tag, receive the change.
the css i used.

.tag-review {
  background-color:red;
}