Maxaud

This plugin allows you to define different CSS rules that get loaded into the page’s for specific devices.
Includes checks for 18 different tablet options and 13 different phone options.
This can be useful if you need to target specific phones or tablets to make CSS changes or show certain advertisements for certain devices.
For more options or if you feel like donating, check out the <a href="http://codecanyon.net/item/mobile-css-pro/3182318/" title="Mobile CSS Pro" rel="nofollow ugc">PRO version of Mobile CSS</a> that is available for purchase.

Mobile CSS

Allows you to define different CSS styles that get loaded for specific mobile devices.

Adnan Shahid

Responsive preview tool for WordPress admin to switch between mobile, tablet, and desktop viewports inside the dashboard.
Key features 
* One-click viewport buttons (Mobile / Tablet / Desktop) 
* Uses an iframe to preview your site inside the WordPress admin 
* No tracking, no external requests, no background processes
<h3>License</h3>
GPLv2 or later.

Responsive Admin Viewports Preview

Responsive preview tool for WordPress admin to switch between mobile, tablet, and desktop viewports inside the dashboard.

Mobile CSS vs Responsive Admin Viewports Preview