i'm looking for a code to have just a border around my entries without the transparent background.

try this:

<style type="text/css">
table.blogbody {
/* this edits the other the place where the blog is */
background-color: transparent;
width:350;
border: 1px solid #FFFFFF;
cellpadding-bottom: 3px;
}
</style>

change the bolded parts, if you want, but maybe you really want to change the FFFFFF because its the code for white.