You can change the default style of WordPress admin area by overriding the old stylesheet. To do this, you have to create a new css file in your theme folder, and then include it in WP-Admin using the admin_head hook.
Add this to
functions.php, change paths and then add the stylesheet in the proper place and add whatever styles you want to override the defaults with, or copy the old stylesheet and use as a template for your new one. Location: /wp-admin/css/colors-fresh.css.