digitalapps

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file. This file is loaded on the frontend. The plugin wp_dequeue_style and wp_deregister_style all other css files. The result is your website only makes one single http request to load css styling. Bonus feature the contents of the css file are minified.
Simply flip the switch and the plugin will do the rest.
NOTE: I have only tested this plugin on my own sites running the AdSense Theme. If you encounter a problem with this plugin please let me know. I will be able to help you.

WP CSS Merge

This plugins detects all of the styles queued via wp_enqueue_style. Contents of each css file are copied and stored in a single css file.

peterpfeiffer

Minify and Optimize your CSS by clicking one button. All you have to do is go to Settings and click CSS Minify. Activating the plugin there will minify and optimize your CSS. No settings required, just click one button.
So anything you can do to speed up the load time of your site should be worth investigating, right? One such thing is “minifying” the CSS files that are used in part to build your site. As is typically the case, there is a plugin available that can do the job for you – Suprise: It’s this one!
Code from this plugin is forked from Autoptimize a really great plugin. You can find it by searching for it here on wordpress.org in the plugin search.
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Upload the zip file and unzip it in the /wp-content/plugins/ directory.
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Activate the plugin through the Plugins menu in WordPress
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Go to Settings and then click CSS Minify and enable the options you want.
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CSS Minify

Way to go, peterpfeiffer! Your plugin contains the same files and functions as in Autoptimize and your plugin's PRO version provides what people could get for free in Autoptimize. Who knew "Copy, cut and paste" could be a business model?!

Autoptimize, but with most features moved to PRO

Minify and Optimize your CSS by clicking one button.

WP CSS Merge vs CSS Minify