Fatal error: Cannot redeclare comment_theme()

Your comments.php is included multiple times for some reason. Look for comments_template() – maybe it is called too early.

Each time that happens, PHP tries to create the function comment_theme() again. This cannot work, function names must be unique.

Move the function declaration to the functions.php. So everything including …

<?php 
function comment_theme($comment, $args, $depth) {
    // functions body   
}
?>

… goes to the functions.php, and the comments.php starts with:

<?php
/*
    The template for displaying Comments.
*/
?>
<div id="comments">