Remove the default WordPress admin-bar.css file and replace it with your own custom one located in your active theme or child theme. You can remove and replace other admin css files too using the same method if you know the css handle.

admin-custom-css.php

<?php 
function sjaved_remove_and_replae_admin_bar_stylesheet(){
	
	//Remove the existing stylesheets
	wp_dequeue_style( 'admin-bar' );
    wp_deregister_style( 'admin-bar' );

    //Enque the custom CSS file from your current theme or child theme
    wp_enqueue_style( 'custom-admin-bar', get_stylesheet_directory_uri() . '/custom-admin-bar.css', false, '1.0.0' );


}

add_action('admin_enqueue_scripts', 'sjaved_remove_and_replae_admin_bar_stylesheet', 10);

Tip! If it does not work at first then try playing with priority parameter in hook (which is 10 at the moment).