Those aren't variables available in the .css templates. You'll need to do something in the postbit template to give the <div> another class if the post userid is the same as the viewer userid ($post['userid'] == $bbuserinfo['userid']) and then you can add some CSS to the additional.css template to set that class background to a certain color.
Could you possibly elaborate a little more on the "give the <div> another class" part?
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Okay so I tried adding a div class to postbit_legacy
<vb:if condition="$post['userid'] == $bbuserinfo['userid']"><div class="postme"></vb:if>
<div class="postrow<vb:if condition="$show['postedited'] || $post['signature']"> has_after_content</vb:if>">
{vb:raw template_hook.postbit_messagearea_start}
<vb:if condition="$post['title'] OR $show['messageicon']">
<h2 class="title icon">
<vb:if condition="$show['messageicon']"><img src="{vb:raw post.iconpath}" alt="{vb:raw post.icontitle}" /> </vb:if>{vb:raw post.title}
</h2>
</vb:if>
<vb:if condition="$post['isfirstshown']">
{vb:raw ad_location.ad_showthread_firstpost_start}
{vb:raw ad_location.thread_first_post_content}
</vb:if>
<vb:if condition="$post['islastshown']">
{vb:raw ad_location.thread_last_post_content}
</vb:if>
<div class="content<vb:if condition="$show['first_ad'] OR $show['last_ad']"> hasad</vb:if>">
<div id="post_message_{vb:raw post.postid}">
<blockquote class="postcontent restore ">
{vb:raw post.message}
</blockquote>
</div>
and the opposite condition for div class="postbody"
<vb:if condition="!$post['userid'] == $bbuserinfo['userid']">
Then I added the postme to the postbit.css
Result was double posts in every single thread! Pretty funny, but not what I was aiming for.