How to add a border or separate each post?

in style.css look for this code around line 343

you have this code

.post, .page {
background:transparent url(images/dot.gif) repeat-x scroll left bottom;
display:block;
margin:1em 0 2em;
padding:0 0 0.6em;

We going make changes as you want it look like your friends

Replace with this

.post, .page {
background:#FFFFFF none repeat scroll 0 0;
border:1px solid #000000;
margin:0 0 5px;
padding:20px 20px 0;
position:relative;
}

I have changed your background which had that doted line image to white added a border and change margins and paddings

now you should have box style posts like your friend has.

You are welcome