The main table contains everything below the dropdown menu and VF banner (ie most of the page).

Make It Disappear ?
Well you could. But if you did then nothing else you write in your profile would show up, so it's probably not advisable.
Hiding the main table isn't a shortcut to getting a nice blank page to write onto- it just means everything under the
banner (including your own styles and content) will be hidden.

Borders
To put a border around it:
<style type="text/css">
.main { border: 5px solid red; }
</style>
[ More Border Styles ]

Background
To set a background color:
<style type="text/css">
.main { background: red; }
</style>
Or a background image:
<style type="text/css">
.main {
background: url('http://www.PATH TO YOUR IMAGE.gif');
background-position: bottom right;
background-repeat: no-repeat;
}
</style>
More on
[ Backgrounds ]

Making Narrow Profiles
To make your profile content all appear in a narrow box, just set the width of .main:
<style type="text/css">
.main { width: 650px; }
</style>
The default position for the narrowed main table is center, but if you want it on the left or right you
can position it with float:
<style type="text/css">
.main {
width: 650px;
float: right;
}
</style>