/**
  * Return compiled css for <head>
  *
  * @since 1.0.0
  *
  * @return string
  */
 public static function get_head_css()
 {
     $css = array();
     $primary_color = Appica_Helpers::get_option('color_primary', '#007aff');
     $success_color = Appica_Helpers::get_option('color_success', '#4cd964');
     $info_color = Appica_Helpers::get_option('color_info', '#5ac8fa');
     $warning_color = Appica_Helpers::get_option('color_warning', '#ffcc00');
     $danger_color = Appica_Helpers::get_option('color_danger', '#ff2d55');
     // Button color
     $css[] = Appica_Helpers::generate_css_rules('.appica-button-color .primary', array('background-color' => $primary_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-button-color .success', array('background-color' => $success_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-button-color .info', array('background-color' => $info_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-button-color .warning', array('background-color' => $warning_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-button-color .danger', array('background-color' => $danger_color));
     // Badge color
     $css[] = Appica_Helpers::generate_css_rules('.appica-badge-color .primary', array('background-color' => $primary_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-badge-color .success', array('background-color' => $success_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-badge-color .info', array('background-color' => $info_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-badge-color .warning', array('background-color' => $warning_color));
     $css[] = Appica_Helpers::generate_css_rules('.appica-badge-color .danger', array('background-color' => $danger_color));
     return implode(PHP_EOL, $css);
 }