Works every time.... just copy/paste onto its own html file...and then, of course, you need to select "Publisher's layout" from Marvin settings otherwise Marvin will ignore most of your styling for it's own. Just open your book in Marvin, tap the screen and go to settings by hitting the Aa in the top right corner, scroll down the settings list a little (below Uniform text size)...
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Cover</title>
</head>
<body style="margin:0; padding:0; text-align:center; background-color:black">
<h1 style="display:none" title="Cover"/>
<div style="margin:0; padding:0">
<svg
xmlns="http://www.w3.org/2000/svg"
height="100vh" width="100vw"
preserveAspectRatio="xMidYMid meet" version="1.1"
viewBox="0 0 1127 1800"
xmlns:xlink="http://www.w3.org/1999/xlink">
<image width="1127" height="1800" xlink:href="../Images/cover.png"/>
</svg>
</div>
</body>