Codex tools: Log in / create account
You can customise the login form using login_head() fairly easily.
Add the following code to functions.php in your theme:
// custom login for theme
function childtheme_custom_login() {
echo '<link rel="stylesheet" type="text/css" href="' . get_bloginfo('stylesheet_directory') . '/customlogin.css" />';
}
add_action('login_head', 'childtheme_custom_login');
This has the effect of adding a reference to a stylesheet to your login form.
You will then need to add a stylesheet called customlogin.css to your theme directory.
For testing purposes, this should start you off:
html {
background-color: #f00;
}
This should produce a login background.