Atomic Variables drupal 8 theme

Hello, I have a issue with some values that I’m getting with hook_form_system_theme_settings_alter. The point is this:
with this hook, I want to create a color picker with:
$form['color'] = array(
'#type' => 'color',
'#title' => t('Color'),
'#default_value' => '#ffffff',
'#description' => t("choose a color")

then, on the admin panel, the admin introduce the value of the color in HEX. 
In a php file, I can obtain the color value with this:

$color = theme_get_setting('color');

but I don’t know how to pass this php variable to a css file for create a variables librarie. 

Drupal version: