In order to assist you properly, please provide more details about your question. However, based on my experience of designing Tumblr themes in the past, the most straightforward approach is to place each post within its own DIV and apply CSS as illustrated below:
Theme Code
<div id="post">
{block:Posts}
{block:Text}
<li class="post text">
{block:Title}
<h3><a href="{Permalink}">{Title}</a></h3>
{/block:Title}
{Body}
</li>
{/block:Text}
{block:Photo}
<li class="post photo">
<img src="{PhotoURL-500}" alt="{PhotoAlt}"/>
{block:Caption}
<div class="caption">{Caption}</div>
{/block:Caption}
</li>
{/block:Photo}
{block:Panorama}
<li class="post panorama">
{LinkOpenTag}
<img src="{PhotoURL-Panorama}" alt="{PhotoAlt}"/>
{LinkCloseTag}
{block:Caption}
<div class="caption">{Caption}</div>
{/block:Caption}
</li>
{/block:Panorama}
{block:Photoset}
<li class="post photoset">
{Photoset-500}
{block:Caption}
<div class="caption">{Caption}</div>
{/block:Caption}
</li>
{/block:Photoset}
{block:Quote}
<li class="post quote">
"{Quote}"
{block:Source}
<div class="source">{Source}</div>
{/block:Source}
</li>
{/block:Quote}
{block:Link}
<li class="post link">
<a href="{URL}" class="link" {Target}>{Name}</a>
{block:Description}
<div class="description">{Description}</div>
{/block:Description}
</li>
{/block:Link}
{block:Chat}
<li class="post chat">
{block:Title}
<h3><a href="{Permalink}">{Title}</a></h3>
{/block:Title}
<ul class="chat">
{block:Lines}
<li class="{Alt} user_{UserNumber}">
{block:Label}
<span class="label">{Label}</span>
{/block:Label}
{Line}
</li>
{/block:Lines}
</ul>
</li>
{/block:Chat}
{block:Video}
<li class="post video">
{Video-500}
{block:Caption}
<div class="caption">{Caption}</div>
{/block:Caption}
</li>
{/block:Video}
{block:Audio}
<li class="post audio">
{AudioPlayerBlack}
{block:Caption}
<div class="caption">{Caption}</div>
{/block:Caption}
</li>
{/block:Audio}
{/block:Posts}
</div>
CSS
#post {
background-image:url('paper.gif');
}
I hope this guide proves useful to you, but it would be beneficial if you could share more specifics regarding your requirements.
If you encounter any difficulties with the theme creation process, feel free to return here with your code or visit this informative link: Creating a custom HTML Tumblr Theme