I had this problem a while back.
The easiest solution would not be with css but with a function that removes that menu item from the admin bar.
Then just add a new menu item with your logo image.
I would do this instead of replacing the icon with your logo with css.
/*Remove WordPress menu from admin bar*/
add_action( 'admin_bar_menu', 'remove_wp_logo', 999 );
function remove_wp_logo( $wp_admin_bar ) {
$wp_admin_bar->remove_node( 'wp-logo' );
}
/*Adds Custom Logo to Admin Bar*/
add_action( 'admin_bar_menu', 'custom_admin_logo', 1 );
//priority 1 sets the location to the front/leftmost of the menu
function custom_admin_logo( $wp_admin_bar ) {
$custom_logo_id = get_theme_mod( 'custom_logo' ); //Uses theme logo
$custom_logo_url = wp_get_attachment_image_url( $custom_logo_id , 'full' );
$args = array(
'id' => 'custom_logo_admin',
'title' => ' ',
'meta' => array( 'html' => '<li id="custom-logo-admin-bar" style="width: 230px;padding: 10px;padding-left: 0px;padding-right: 25px;"><img class="overlay" src="'.$custom_logo_url.'" style="float: left;width: 100%;height: auto;"></li>' )
);
$wp_admin_bar->add_node( $args );
}
You can style your image with css either in your stylesheet or directly here in the meta array.
The $wp_admin_bar->add_node( $args ); is what actually adds the new node into the admin bar.
P.S. some of the styling here is just what I needed for my own purposes, feel free to change.