Add the following to the functions.php

function mytheme_costumizer_register($wp_customize) {
	$wp_customize->add_section('theme_colors', array(
	'title' => __('Colors', 'blankslate'),
	'description' => 'Modify the website colors'
	));
	$wp_customize->add_setting('dark_backgrounds', array(
	'default' => '#302d2c',
	));
	$wp_customize->add_setting('light_backgrounds', array(
	'default' => '#ffffff',
	));
	$wp_customize->add_control( new WP_Customize_Color_Control($wp_customize,'dark_backgrounds', array(
	'label' => __('Edit Dark Backgrounds', 'blankslate'),
	'section' => 'theme_colors',
	'settings' => 'dark_backgrounds'
	) ));
	$wp_customize->add_control( new WP_Customize_Color_Control($wp_customize,'light_backgrounds', array(
	'label' => __('Edit light Backgrounds', 'blankslate'),
	'section' => 'theme_colors',
	'settings' => 'light_backgrounds'
	) ));
}
add_action('customize_register', 'mytheme_costumizer_register');

Then call the CSS

function mytheme_css_customizer() {
	?>
	<style type="text/css">
		.dark_background {
			background: <?php echo get_theme_mod('dark_backgrounds'); ?>;
		}
.light_background {
			background: <?php echo get_theme_mod('light_backgrounds'); ?>;
		}
	</style>
<?php		
}
add_action('wp_head', 'mytheme_css_customizer' );