C9 Rendering Issue

I've noticed that when I'm signed into c9 everything is displayed left aligned and fits to the width of my screen, while when I'm signed out of c9 everything is centred and fixed width. I think this may be to do with custom CSS (reproduced below) in use on my profile, but I don't know enough about CSS to be sure, nor to know whether the CSS is at fault, or whether c9 is at fault. I'm using Opera 11.62 on Vista Home Premium SP2.

When I'm logged out.

 When I'm logged in. Although it's clearly not designed to be viewed like this I kind of like it.

Custom CSS code:

body 
{ 
background-color: #FEFEFE; 
} 
.branding .toolbar 
{ 
background-image: url("http://images.callemin.be/c9/background.png"); 
background-position: center; 
border-color: #f08614; 
border-width: 4px; 
} 
.branding .navigation .logo a:hover{ 
background-image: url("http://images.callemin.be/c9/logo_hover.png"); 
  
} 
.mainHolder 
  
  
{ 
width: 90%; 
} 
.posts { 
float: none; 
} 
.posts > LI{ 
width: 90%; 
} 
.posts LI .post{ 
width: 85%; 
} 

@GoddersUK: The width: 90%; in the mainholder section would be stretching it out. It looks like the image is not centered, so it just moves to the left. There's no CSS class defined for the image, so it doesn't look like it can be centered.

I don't think it's the image that's actually causing the issue (EDIT: Assuming that by "image" you mean the big yellow Windows' Dev Centre one), as it appears on pages without the issue too (see screenshot), although it does make it obvious - most stuff below the top menu bar seems to be left aligned. I did try tweaking the width of .mainHolder but it's so zoom/window size/screen res dependant it's not really worth it (fwiw 67.5% seemed about right for Firefox 11.0 at 100% zoom on my 1440*900 laptop screen).

 EDIT: Excessively zoomed for clarity