I found this in the preview file of the css folder in the calendar

@CHARSET "UTF-8";
/* reset */
html{overflow-x: hidden; overflow-y: scroll; color:#000;background:#FFF;}body,div,dl,dt,dd,ul,ol,li,h1,h2,h3,h4,h5,h6,pre,code,form,fieldset,legend,input,button,textarea,p,blockquote,th,td{margin:0;padding:0;}table{border-collapse:collapse;border-spacing:0;}fieldset,img{border:0;}address,caption,cite,code,dfn,em,strong,th,var,optgroup{font-style:inherit;font-weight:inherit;}del,ins{text-decoration:none;}li{list-style:none;}caption,th{text-align:left;}h1,h2,h3,h4,h5,h6{font-size:100%;font-weight:normal;}q:before,q:after{content:'';}abbr,acronym{border:0;font-variant:normal;}sup{vertical-align:baseline;}sub{vertical-align:baseline;}legend{color:#000;}input,button,textarea,select,optgroup,option{font-family:inherit;font-size:inherit;font-style:inherit;font-weight:inherit;}input,button,textarea,select{*font-size:100%;}
/* general */
html, body{
background-color: #fff;
color: #000;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
}
#container{
margin: 0 auto;
width: 1000px;
overflow: hidden;
padding: 10px;
}
#nav{
background-color: #ddd;
padding: 5px 10px;
margin: 5px 0;
}

Is this what I need to change?

Yup.
Start with the #container there then go and look in the files for anything that says <div id="container"> and change those also. Remember though that every time you make any changes to your calendar your going to need to do this. I