If you want to add inline CSS, use wp_add_inline_style()
.
<?php
/**
* Plugin Name: WPSE 257470
* Description: WordPress StackExchange question 257470
* Plugin URI: https://wordpress.stackexchange.com/questions/257470/
* Author: Nathan Johnson
* Licence: GPL2+
* Licence URI: https://www.gnu.org/licenses/gpl-2.0.en.html
*/
//* Don't access this file directly
defined( 'ABSPATH' ) or die();
if( ! class_exists( 'wpse_257470' ) ):
class wpse_257470 {
public function plugins_loaded() {
add_action( 'wp_enqueue_scripts', [ $this, 'wp_enqueue_scripts' ] );
}
public function wp_enqueue_scripts() {
//* It's necessary to have a 'base' CSS file that we can attach to
wp_enqueue_style( 'wpse-257470', plugin_dir_url( __FILE__ ) . 'style.css' );
$color="#257470";
$custom_css = ".custom_color { color: $color };";
wp_add_inline_style( 'wpse-257470', $custom_css );
}
}
add_action( 'plugins_loaded', [ new wpse_257470(), 'plugins_loaded' ] );
endif;