Dojo Digital

This plugin adds a textarea to the Customize page that allows theme editors to write, preview, and implement CSS code in real time. 
It also includes a frontend CSS editor window with live preview capability.

Live CSS Preview

Live front-end and Customizer CSS editing powered by native WordPress Additional CSS storage.

Mark Jaquith

This plugin allows you to customize the logo on the WordPress login screen. There is zero configuration. You just drop the logo file into your WordPress content directory, named <code>login-logo.png</code> and this plugin takes over.
Note that you should use a transparent background on the PNG image, crop it tightly (no padding pixels) and use a width of exactly 312 pixels for best results. Wider images will be downscaled in modern browsers, but it isn’t recommended to rely on that.
This plugin also works in the <code>mu-plugins</code> directory.

Login Logo

Customize the logo on the WP login screen by simply dropping a file named login-logo.png into your WP content directory. CSS is automatic!

Live CSS Preview vs Login Logo