it's because of the css you added to CSS Inserts
HTML Code:
/*BELOW IS CODE FOR THE POST TITLE IMAGE*/
.date {
background: url('http://wordthirst.com/wp-content/themes/atahualpa/images/post-title-date-img1.gif') repeat-x scroll top left;
display: inline;
float: left;
width:55px;
text-align:center;
color:#002157;
font-family: georgia, Arial, Helvetica, sans-serif;
font-weight: bold;
font-size:12px;
margin: 0px 10px -10px 0px;
}
this is very global and since the <body...> html statement gets a class of 'date' assigned to it, this CSS applies so you see a new page background.