This can be done with your current CSS.
Please post your post css. Your comment.php
Basically with your css stylesheet if you want to try it yourself, to give the ul within that div it's own property simply have this.
For example only.
#comment {some style info}
#comment .u-list {
width:75%;
}
#comment .u-list ul {
font-size:12px;
}
#comment .u-list li {
more styling.
}
<div id="comment">
<div class="u-list">
<ul>
<li></li>
<li></li>
<li></li>
<ul>
</div>
</div>
Make some sense? I'm trying to track down a better example. A plugin is really not the answer. You may have to edit your functions also if your adding a div, just so everything gets styled accordingly.