Creating a boarder around all posts with child

Hello i would like to create a solid black boarder around every single post. I have a child theme! Below i have included some of my style.css which may relate to this..

Thanks!

.sticky {
}
.entry-title, .entry-title a{
text-decoration:none;
font-size:38px;
color:#000;
text-transform:capitalize;
text-align:center;
}
.entry-title a:hover{
text-decoration:none;
color:#666;
}
.archive-title{
margin-bottom:15px;
font-size:22px;
text-transform:uppercase;
color:#999;
text-align:center;
}
.hentry {
margin:0 0 80px 0;
padding:0 0 80px;
border-bottom:4px double #e6e6e6;
}
.entry-meta {
clear: both;
text-align:center;
font-family: georgia, serif;
font-style:italic;
margin:20px 0;
}
.entry-meta a{
text-decoration:none;
color:#999;
}
.sep{
color:#e6e6e6;
font-style:italic;
}
.byline {
display: none;
}
.single .byline,
.group-blog .byline {
display: inline;
}
.entry-content,
.entry-summary {
margin: 1.5em 0 0;
}
.page-links {
clear: both;
margin: 0 0 1.5em;
}
.single-thumbnail{
margin:20px 0;
}
.single-thumbnail img:hover{
filter:alpha(opacity=90);
-moz-opacity:0.9;
-khtml-opacity: 0.9;
opacity: 0.9;
}