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.

Khang Minh

Allows you to combine and minify your CSS and JS files to improve page load time. This plugin uses the PHP library <a href="http://code.google.com/p/minify/" rel="nofollow ugc">Minify</a> and relies on WordPress’s enqueueing system rather than the output buffer, which respects the order of CSS and JS files as well as their dependencies. BWP Minify is very customizable and easy to use.
Useful resources to help you get started and make the most out of BWP Minify
<ul>
<li><a href="http://betterwp.net/wordpress-plugins/bwp-minify/#usage" rel="nofollow ugc">Official Documentation</a></li>
<li><a href="http://betterwp.net/wordpress-minify-javascript-css/" rel="nofollow ugc">WordPress Minify Best Practices</a></li>
</ul>
Some Features
<ul>
<li>Uses enqueueing system of WordPress which improves compatibility with other plugins and themes</li>
<li>Allows you to move enqueued files to desired locations (header, footer, oblivion, etc.) via a dedicated management page</li>
<li>Allows you to change various Minify settings (cache directory, cache age, debug mode, etc.) directly in admin</li>
<li>Allows you to use friendly Minify urls, such as <code>http://example.com/path/to/cache/somestring.js</code></li>
<li>Allows you to use CDN for minified contents, one CDN host for JS and one for CSS with SSL support</li>
<li>Allows you to split long Minify strings into shorter ones</li>
<li>Offers various way to add a cache buster to your minify string such as WordPress’s version, Theme’s version, Cache folder’s last modified timestap, etc.</li>
<li>Supports script localization (<code>wp_localize_script()</code>)</li>
<li>Supports inline styles</li>
<li>Supports RTL stylesheets</li>
<li>Supports media-specific stylesheets (e.g. ‘screen’, ‘print’, etc.)</li>
<li>Supports conditional stylesheets (e.g. <code><!--[if lt IE 7]></code>)</li>
<li>Provides hooks for further customization</li>
<li>WordPress Multi-site compatible</li>
</ul>
Please don’t forget to rate this plugin <a href="https://wordpress.org/support/view/plugin-reviews/bwp-minify?filter=5" rel="ugc">5 shining stars</a> if you like it, thanks!
Get in touch
<ul>
<li>Support is provided via <a href="http://betterwp.net/community/" rel="nofollow ugc">BetterWP.net Community</a>.</li>
<li>Follow and contribute to development via <a href="https://github.com/OddOneOut/Better-WordPress-Minify" rel="nofollow ugc">Github</a>.</li>
<li>You can also follow me on <a href="http://twitter.com/0dd0ne0ut" rel="nofollow ugc">Twitter</a>.</li>
<li>Check out <a href="http://feeds.feedburner.com/BetterWPnet" rel="nofollow ugc">latest WordPress Tips and Ideas</a> from BetterWP.net.</li>
</ul>
Languages
<ul>
<li>English (default)</li>
<li>Romanian (ro_RO) – Thanks to <a href="www.enjoyprepaid.com" rel="nofollow ugc">Luke Tyler, International Calling Cards</a>!</li>
<li>Turkish (tr_TR) – Thanks to Hakan E</li>
<li>French (fr_FR) – Thanks to Sebastien</li>
<li>Italian (it_IT) – Thanks to Gabriele – http://cookspot.it</li>
<li>Spanish (es_ES) – Thanks to Ruben Hernandez – http://usitility.com/</li>
<li>Dutch (nl_NL) – Thanks to Martijn van Egmond</li>
<li>German (de_DE) – Thanks to Matthias</li>
<li>Serbo-Croatian (sr_RS) – Thanks to Borisa Djuraskovic – <a href="http://www.webhostinghub.com/" rel="nofollow ugc">Web Hosting Hub</a></li>
<li>Indonesian (id_ID) – Thanks to Nasrulhaq Muiz – http://al-badar.net</li>
<li>Russian (ru_RU) – Thanks to Эдуард Валеев</li>
</ul>
Please <a href="http://betterwp.net/wordpress-tips/create-pot-file-using-poedit/" rel="nofollow ugc">help translate</a> this plugin!

Better WordPress Minify

Allows you to combine and minify your CSS and JS files to improve page load time.

WP CSS Merge vs Better WordPress Minify