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.

Blogestudio

Pau Iglesias

Improve user experience by having your above-the-fold (top of the page) CSS styles in-page.
Even if the rest of the CSS files take a seconds to load, these specific CSS styles displayed from the head section ensure a quick rendering of your page and better score in testing systems like Google Page Speed Insights.
But you do not need to maintain two separate stylesheets, just select specific fragments of your theme style file with a special markup to create an above-the-fold styles joining the CSS fragments in the head section.
This plugin enables a special open and close tags that you can insert editing your style.css theme file to surround pieces of code:
[css-above-the-fold]
… Your theme CSS code fragment …
[/css-above-the-fold]
But this syntax is not an standard CSS, so you need to include this tags between CSS comments, there are two ways to do it:
<h3>1. Comment whole section</h3>
The easiest way, the CSS is rendered only in the head but not in the stylesheet. 
This kind of markup does not allow to use your own CSS comments inside the fragment. 
Take care to use this way only when the plugin is active, because you are commenting some parts of your theme stylesheet.
/* [css-above-the-fold]
… CSS code fragment …
[/css-above-the-fold] */
<h3>2. Comment only the tags</h3>
The unobtrusive way, it allows you to include comments inside fragments, but the fragments are rendered both in the head section and the CSS file. 
This way is plugin-independent, and your theme stylesheet will keep running with this plugin activated or not.
/* [css-above-the-fold] */
… CSS code fragment …
/* [/css-above-the-fold] */
You can use these two kinds of markup combined in your CSS File.
The resulting CSS fragments introduced in the header are compacted and minified.
This plugin only read the style.css file when it is modified, and stores the results using the WP options API.

CSS Above The Fold

Good idea for a plugin. Works perfectly. Thanks.



Good idea

Faster CSS browser rendering displaying selected fragments of your theme stylesheet file directly into the head section.

WP CSS Merge vs CSS Above The Fold