You'll need to edit your CSS definitions by adding this after the rest of the code in your AdminCP > Style Manager > All Style Options:
.info {
padding-left: 3px;
padding-right: 0px;
padding-top: 3px;
padding-bottom: 3px;
background-color: #F1F1F1;
border-top: 1px solid #DBDBDB;
border-right: 2px solid #DBDBDB;
border-left: 1px solid #DBDBDB;
border-bottom: 2px solid #DBDBDB;
margin-bottom: 3px;
}
Edit the color codes as you see fit. Lastly, you'll need to edit your postbit template to include the new info tag. For instance:
<if condition="$post['joindate']"><div>$vbphrase[join_date]: $post[joindate]</div></if>
Becomes:
<if condition="$post['joindate']"><div class="info">$vbphrase[join_date]: $post[joindate]</div></if>
That should be it ;)