style.css

/**
 * Theme Name: BP FB Style Activity Commenting Demo
 * Theme URI: http://buddydev.com
 * Version: 1.0
 * Description: Facebook style comment demonstration Theme
 * Author: Brajesh Singh
 * Tags:  buddypress
 * Template: bp-default
*/

/*FB style activity comment form css*/
/*
If you want to make the form visible for all activities, remove the li.has-comments from the first line below */


li.has-comments div.activity-comments form.ac-form {
    display:block;
    margin:0 0 5px 0;
}
.ac-form .ac-reply-avatar{
    display:none;
}

div.activity-comments form div.ac-reply-content{
    margin-left: 0;
    padding-left: 0;
    margin-bottom: 5px;
     height:35px;/* you may need to adjust height according to the one you choose for your textarea*/
}

div.activity-comments form textarea {
    height: 13px;
}
div.activity-comments form input[type='submit']{
    display:none;
}

/*on active/focus*/
form.active .ac-reply-avatar{
    float:left;
    display: block;
}
form.active .ac-reply-avatar img.avatar {
    float: left;
    margin: 0 3px 0 0;
     height: 25px;
    
    width: 25px;
}
div.activity-comments form.active div.ac-reply-content{
    height:auto;
}
form.active .ac-textarea{

    margin-bottom: 10px;
    padding: 8px;
    padding-top:0;
    }
    div.activity-comments form.active div.ac-reply-content{
    margin-left: 30px;
    padding-left: 2px;
    
}