You need a 100% width table in there instead of applying the background color to the body tag.
Here are the 2 setups I use that take control the forwarding background color (#FFFFFF), while setting the html section background (#252525).
Fixed Width Email:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title></title>
<style type="text/css">
/* Client-specific Styles */
#outlook a {padding:0;}
body{width:100% !important; -webkit-text-size-adjust:100%; -ms-text-size-adjust:100%; margin:0; padding:0;} /* force default font sizes */
.ExternalClass {width:100%;} .ExternalClass, .ExternalClass p, .ExternalClass span, .ExternalClass font, .ExternalClass td, .ExternalClass div {line-height: 100%;} /* Hotmail */
a:active, a:visited, a[href^="tel"], a[href^="sms"] { text-decoration: none; color: #000001 !important; pointer-events: auto; cursor: default;}
table td {border-collapse: collapse;}
</style>
</head>
<body leftmargin="0" topmargin="0" marginwidth="0" marginheight="0" style="margin: 0px; padding: 0px; background-color: #FFFFFF;" bgcolor="#FFFFFF"><table bgcolor="#252525" width="100%" border="0" align="center" cellpadding="0" cellspacing="0"><tr><td><table width="600" border="0" align="center" cellpadding="0" cellspacing="0"><tr><td valign="top" style="padding-top:30px; padding-bottom:30px;">
<table width="100%" border="0" cellpadding="0" cellspacing="0" bgcolor="white">
<tr>
<td style="padding:30px;">
Content
</td>
</tr>
</table>
</td></tr></table></td></tr></table></body></html>
Fluid Email:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title></title>
<style type="text/css">
/* Client-specific Styles */
#outlook a {padding:0;}
body{width:100% !important; -webkit-text-size-adjust:100%; -ms-text-size-adjust:100%; margin:0; padding:0;} /* force default font sizes */
.ExternalClass {width:100%;} .ExternalClass, .ExternalClass p, .ExternalClass span, .ExternalClass font, .ExternalClass td, .ExternalClass div {line-height: 100%;} /* Hotmail */
a:active, a:visited, a[href^="tel"], a[href^="sms"] { text-decoration: none; color: #005288 !important; pointer-events: auto; cursor: default;}
table td {border-collapse: collapse;}
@media only screen and (min-width: 600px) { .maxW { width:600px !important; } }
</style>
</head>
<body style="margin: 0px; padding: 0px; -webkit-text-size-adjust:none; -ms-text-size-adjust:none;" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0" bgcolor="#FFFFFF"><table bgcolor="#252525" width="100%" border="0" align="center" cellpadding="0" cellspacing="0"><tr><td valign="top">
<!--[if (gte mso 9)|(IE)]>
<table width="600" align="center" cellpadding="0" cellspacing="0" border="0"><tr><td valign="top">
<![endif]-->
<table width="100%" class="maxW" style="max-width: 600px; margin: auto;" border="0" align="center" cellpadding="0" cellspacing="0"><tr><td valign="top" align="center" style="padding-top:30px; padding-bottom:30px;">
<table width="100%" border="0" cellpadding="0" cellspacing="0" bgcolor="#FFFFFF" style="margin: auto;">
<tr>
<td align="left" style="padding:30px;">
Content
</td>
</tr>
</table>
</td></tr></table>
<!--[if (gte mso 9)|(IE)]>
</td></tr></table>
<![endif]-->
</td></tr></table></body></html>