When using the aero theme the content area of a MessageBox has a different background color depending on value of modal. When modal is set to true the background is set to #C4D2E3 because of an extra style, when modal is false it uses #CFE0F5 (which looks like the Ext Blue theme).

modal:true
Code:
ytheme-aero.css (line 284)
body.x-masked #x-msg-box .x-dlg-bd, body.x-body-masked #x-msg-box .x-dlg-bd {
 background:#C4D2E3 none repeat scroll 0%;
 border:0pt none;
}

ytheme-aero.css (line 280)
#x-msg-box .x-dlg-bd {ytheme-aero.css (line 280)
 background:#CFE0F5 none repeat scroll 0%;
 border:0pt none;
}
modal:false;
Code:
ytheme-aero.css (line 280)
#x-msg-box .x-dlg-bd {
 background:#CFE0F5 none repeat scroll 0% 50%;
 border:0pt none;
}