look in themes/flow/styles/flow-theme.css
.login-page .login-header {
position: relative;
background: #373e4a;
padding: 100px 0;
-moz-transition: all 550ms ease-in-out;
-o-transition: all 550ms ease-in-out;
-webkit-transition: all 550ms ease-in-out;
transition: all 550ms ease-in-out;
}
just change background to whatever you want, color or image
however the above is only going to change the top half of the login page because the div is wrapped in class="page-body which has a #303641 background declared in the aforementioned css file, but i think page body is declared elsewhere in the script so if i was you i'd just set the background in the login-header and get rid of the page-body reference in the .html file
this should work, if not let me know n i cant test it out on a test page.