Tesselate theme: Open post to same window??

Hi, I am putting together a Tumblr for a friend: www.stephaniefarouze.tumblr.com I cannot for the life of me get each post to open to a simple "preview" type window like most other themes

Here, a colleague of ours has used the same Tesselate theme, and her posts "pop open" like we want: http://celineexbrayatmakeup.tumblr.com/

How do we go about editing the HTML to force the posts to open in this way??

Thanks in advance for your assistance !