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Plugin API/Action Reference/login head

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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.

Further information.