inline-css-wordpress.php

/**
 * Read the contents of the theme CSS file into a variable and ouput.
 **/
function ajo_inline_styles() {
	$css_file = get_stylesheet_uri(); // This will load the entire theme stylesheet in the head (only recommended if it's small)
	// to include specific CSS use: `get_template_directory_uri() . '/path-to-above-the-fold.css'
	
	$css = file_get_contents($css_file);

	echo "<style>{$css}</style>";
}

// Run the function in the wp_head action
add_action( 'wp_head', 'ajo_inline_styles', 40 );

Just updated to include some comments about including the entire theme directory CSS vs. an "above the fold" CSS file.