Get path to themes CSS file inside of a Plugin

Get the absolute path: get_stylesheet_directory()

Get the URI: get_stylesheet_directory_uri()

Edit: Here’s a proof of concept plugin. It will just print out the stylesheet path and uri in the head tag of your site (front end). If these functions are not working in your plugin, there might be some other factor not explained in the OP. Please post your code.

<?php
/**
 * Plugin Name: Get Style Sheet Test
 * Plugin URI: 
 * Description: Demonstration of get_stylesheet_directory() and get_stylesheet_directory_uri() within a plugin.
 * Version: 1.0
 * Author: goto10
 * Author URI: 
 * License: 
 */


add_action( 'wp_head', 'get_stylesheet_dir_and_uri_test' );

function get_stylesheet_dir_and_uri_test() {
    $output = "<!-- \nGet the absolute path using get_stylesheet_directory(): " . get_stylesheet_directory() . "\n";
    $output .= "Get the URI using get_stylesheet_directory_uri(): " . get_stylesheet_directory_uri() . "\n -->";
    echo $output;
}