In the Control Panel under Modules there is one for JavaScript and CSS. That's where these things can be changed... once you know how.
With the development tools - element inspector of my browser I see the following structure of the login page:
Code:
<head>
...
</head>
<body class="wt-global wt-theme-webtrees wt-route-LoginPage">
...
</body>
A good candidate for a background on the page is class wt-route-LoginPage. I think you need to do something alike this CSS styling:
Code:
.wt-route-LoginPage: {
background-image: url("/public/login-background.png");
}
