Reply To: add a specific class

You can also use this PHP function to add a class to the forum instead of doing it manually.

function rkk_add_forum_classes( $classes ) {
 $classes[] = 'class';
 return $classes;
}
add_filter( 'bbp_get_forum_class','rkk_add_forum_classes' );

Now I would like to change while loop_forums in index forums on loop-forums.php

I do not know exactly what you are trying to do, but for just customizing it you can put the loop-forums.php file in a child theme in a folder called bbpress and then customize the file to your liking.

https://codex.wordpress.org/Child_Themes