Full screen page background image?

I know it is bad to change a plugin’s source code but I use it just on a temporary site. So in er-custom-login.php I changed line 107 in

body.login{
background: <?php echo $er_top_bg_color ?> url(<?php echo $er_top_bg_image ?>) no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(src='<?php echo $er_top_bg_image ?>', sizingMethod='scale');
-ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='<?php echo $er_top_bg_image ?>', sizingMethod='scale')";
}