its done with Cascading Style Sheets CSS for short, basically with CSS you can alter just about anything. one thing you can alter are single borders of a element or container, so here they would have just made one border solid eg.
.rightside { border-right-style : solid; border-right-width : 5px; }
this will give anything using the class rightside a solid righthand border 5 pixels thick
that is done like this
<div class="rightside">
anyway read up more about it at http://www.w3schools.com/css/default.asp
You shouldn't design for designers, or design for your boss. Good design is about problem-solving under constraints - design has to have a goal of achieving some purpose. Jakob Nielsen