Image Background Post Problem in Ported IPB theme

Gradient/Image Post for staffs don't work on a customized theme. I have a punbb version of forum and I used the code below:

<span class="a">Administrator</span>

Javascript:

Code:

$(function() {
$('.a').closest('.post').addClass('b');
});

Css:

Code:

.post.online.b {
background: url("http://i.imgur.com/0AwFcJj.png") repeat-y scroll left top, url("IMAGE_URL") no-repeat scroll right top #E5F1F9;
}
.post.b {
background: url("http://i.imgur.com/0AwFcJj.png") repeat-y scroll left top #E5F1F9;
}

I am using concise theme and it seems that the above code doesn't work with it since it was modified.
( Theme Preview: http://code-testing.forumtl.com/ )

I hope you can help me find a way to work staff background post in the said theme. Thanks in advance.

Result Preview:

Preview: