A drop-in (mu-plugin or include) helper for adding dynamic styles to a template.

dynamic-styles.php

<?php
/**
 * Plugin name: Dynamic Styles
 * Plugin URI: https://gist.github.com/crstauf/20479cb9d1dbcd59fa81647aae484cd9
 * Author: Caleb Stauffer
 * Author URI: https://develop.calebstauffer.com
 * Version: 1.1
 */

/**
 * Class: DynamicStyles
 */
class DynamicStyles {

	/**
	 * @var string[] $registered Registered dynamic styles (handle => selectors and properties).
	 * @var string[] $queue Handles to print.
	 * @var array $done Handles printed (handle => microtime)
	 */
	public $registered = array();
	public $queue = array();
	public $done = array();

	/**
	 * Get or initialize instance.
	 *
	 * @return self
	 */
	static function instance() {
		static $_instance = null;

		if ( is_null( $_instance ) )
			$_instance = new self;

		return $_instance;
	}

	/**
	 * Construct.
	 */
	protected function __construct() {

		add_action( 'wp_print_styles',         array( $this, 'action__wp_print_styles' ) );
		add_action( 'wp_print_footer_scripts', array( $this, 'action__wp_print_footer_scripts' ) );

		add_action( 'dynamic-styles/print_queued_items', array( $this, 'print_queued_items' ) );

	}

	/**
	 * Register styles.
	 *
	 * @param string $handle
	 * @param string $selectors
	 * @param string $properties
	 * @uses self::instance()
	 */
	static function register( $handle, $selectors, $properties ) {
		$self = static::instance();

		if ( isset( $self->registered[$handle] ) )
			return;

		$self->registered[$handle] = $selectors . ' { ' . $properties . ' }';
	}

	/**
	 * Enqueue styles.
	 *
	 * @param string $handle
	 * @param string $selectors
	 * @param string $properties
	 * @uses self::register()
	 * @uses self::instance()
	 */
	static function enqueue( $handle, $selectors = '', $properties = '' ) {
		if (
			   !empty( $selectors )
			&& !empty( $properties )
		)
			static::register( $handle, $selectors, $properties );

		$self = static::instance();

		if (
			  !isset( $self->registered[$handle] )
			|| isset( $self->done[$handle] )
		)
			return;

		$self->queue[] = $handle;
	}

	/**
	 * Print items.
	 *
	 * @param string|array $handles
	 * @return array
	 */
	static function print_items( $handles ) {
		$handles = ( array ) $handles;
		$self = static::instance();

		$handles = array_filter( $handles, function( $handle ) {
			return (
				    isset( $self->registered[$handle] )
				&& !isset( $self->done[$handle] )
			);
		} );

		do_action( 'dynamic-styles/printing_items', $handles );

		# Open style block.
		echo '<style type="text/css">' . "\n";

			# Start items timer.
			do_action( 'qm/start', ( $timer = 'dynamic-styles/printing_items' ) );

			# Print items.
			foreach ( $handles as $handle ) {
				echo $self->registered[$handle] . "\n";

				$self->done[$handle] = microtime( true );

				# Lap items timer.
				do_action( 'qm/lap', $timer );
			}

			# Stop items timer.
			do_action( 'qm/stop', $timer );

		# Close style block.
		echo '</style>' . "\n";

		do_action( 'dynamic-styles/printed_items', $handles );
	}

	/**
	 * Print queued styles.
	 *
	 * @uses self::print_items()
	 */
	function print_queued_items() {
		# Store queue.
		$queue = $this->queue;

		do_action( 'dynamic-styles/printing_queued_items', $queue );

		static::print_items( $queue );

		foreach ( $queue as $i => $handle )
			if ( isset( $this->done[$handle] ) )
				unset( $this->queue[$i] );

		do_action( 'dynamic-styles/printed_queued_items', $queue );
	}

	/**
	 * Action: wp_print_styles
	 *
	 * - print queued styles in header
	 */
	function action__wp_print_styles() {
		if ( empty( $this->queue ) )
			return;

		echo '<!-- Begin header dynamic styles -->' . "\n";
		do_action( 'dynamic-styles/print_queued_items' );
		echo '<!-- End header dynamic styles -->' . "\n";
	}

	/**
	 * Action: wp_print_footer_scripts
	 *
	 * - print queued styles in footer
	 */
	function action__wp_print_footer_scripts() {
		if ( empty( $this->queue ) )
			return;

		echo '<!-- Begin footer dynamic styles -->' . "\n";
		do_action( 'dynamic-styles/print_queued_items' );
		echo '<!-- End footer dynamic styles -->' . "\n";
	}

}

if ( !function_exists( 'register_dynamic_style' ) ) {

	/**
	 * Register styles.
	 *
	 * @param string $handle
	 * @param string $selectors
	 * @param string $properties
	 * @uses DynamicStyles::register()
	 */
	function register_dynamic_style( $handle, $selectors, $properties ) {
		DynamicStyles::register( $handle, $selectors, $properties );
	}

}

if ( !function_exists( 'enqueue_dynamic_style' ) ) {

	/**
	 * Enqueue styles.
	 *
	 * @param string $handle
	 * @param string $selectors
	 * @param string $properties
	 * @uses DynamicStyles::enqueue()
	 */
	function enqueue_dynamic_style( $handle, $selectors = '', $properties = '' ) {
		DynamicStyles::enqueue( $handle, $selectors, $properties );
	}

}

?>