To begin with, I will say that there is a great plugin Stick Admin Bar To Bottom for these purposes. The plugin itself is written very simply., just two pieces of code.

Stick Admin Bar To Bottom plugin code:

/*
Plugin Name: Stick Admin Bar To Bottom
Description: Annoyed by the new Admin Bar that is covering the top 28 pixels of your website, but you don't want it completely gone? This plugin sticks the Admin Bar to the bottom of your screen!
Author: Coen Jacobs
Version: 1.2
Author URI: https://coenjacobs.me
*/
function stick_admin_bar_to_bottom_css() {
	$version = get_bloginfo( 'version' );

	if ( version_compare( $version, '3.3', '<' ) ) {
		$css_file = 'wordpress-3-1.css';
	} else {
		$css_file = 'wordpress-3-3.css';
	}
	wp_enqueue_style( 'stick-admin-bar-to-bottom', plugins_url( 'css/' . $css_file, __FILE__ ) );
}

add_action( 'admin_init', 'stick_admin_bar_to_bottom_css' );
add_action( 'wp_enqueue_scripts', 'stick_admin_bar_to_bottom_css' );

Style code wordpress-3-3.css:

* html body {
    margin-top: 0 !important; 
}

body.admin-bar {
	margin-top: -28px;
	padding-bottom: 28px;
}

body.wp-admin #footer {
	padding-bottom: 28px;
}

#wpadminbar {
	top: auto !important;
	bottom: 0;
}

#wpadminbar .quicklinks .ab-sub-wrapper {
	bottom: 28px;
}

#wpadminbar .quicklinks .ab-sub-wrapper ul .ab-sub-wrapper {
	bottom: -7px;
}

Style code wordpress-3-1.css:

* html body {
    margin-top: 0 !important; 
}

body.admin-bar {
	margin-top: -28px;
	padding-bottom: 28px;
}

body.wp-admin #footer {
	padding-bottom: 28px;
}

#wpadminbar {
	top: auto !important;
	bottom: 0;
}

#wpadminbar .quicklinks .menupop ul {
	bottom: 28px;
}

In the plugin itself, the code is placed in the main file called stick-admin-bar-to-bottom.php, folder next to it css and in it styles for admin bar file called wordpress-3-3.css and wordpress-3-1.css.

Laid out exclusively for myself and you as an example of the implementation of such code. You can safely use the plugin itself.