Ashok

Adminpress is a very simple plugin that will help to customize the WordPress admin panel. You will have lots of options and those are very easy to use. The plugin is very user friendly.
Features 
1. Common Styles 
2. Menu Styles 
3. Login page styles 
4. Menu Management 
5. Dashboard Widget 
6. Custom Admin Bar 
7. Footer Text 
8. Replace text 
9. Ajax Search 
10. Custom css and js

Adminpress – WordPress Admin Theme

vegacorp

Jose Vega

Beautiful design for WP Admin, you can use this plugin to simplify the looks. The WP Admin will look very clean and simple.
You can install the plugin and it works automatically.
If you don´t like the main color, you can change it in Settings > General options.
<h4>Multisite compatible.</h4>
If you define a constant in wp-config.php, the setting will be applied to all sites automatically and the options field won´t be displayed: <code>define('WPCAT_MAIN_COLOR', '#000');</code>
<h4>Recommended plugins</h4>
<ul>
<li>WP Sheet Editor is a free plugin that allows you to edit WordPress posts, pages, and WooCommerce products using a spreadsheet.</li>
</ul>
<a href="https://wordpress.org/plugins/wp-sheet-editor-bulk-spreadsheet-editor-for-posts-and-pages/" rel="ugc">Download “wp sheet editor” plugin</a>

Clean WP Admin Theme – Simple design

This plugin will make wp-admin look cleaner. It will look "simpler" for your clients.

Useful for clients sites

Beautiful design for WP Admin, Clean Admin Theme for wp-admin.

Adminpress – WordPress Admin Theme vs Clean WP Admin Theme – Simple design