WordPress how to pass Inline CSS to existing plugin or theme

inline-css-wordpress.php

<?php

//Adding CSS inline style to an existing CSS stylesheet
function wpb_add_inline_css() {

        //All the user input CSS settings as set in the plugin settings
        $slicknav_custom_css = "
            @media screen and (max-width: {$ng_slicknav_width}px) {
                {$ng_slicknav_menu} {
                  display: none;
               }
               .slicknav_menu {
                  display: block;
                  background: {$ng_slicknav_background};
               }
               .slicknav_btn {
                  background-color:{$ng_slicknav_button};
                  float:{$ng_slicknav_button_position};
               }

           }";

  //Add the above custom CSS via wp_add_inline_style
  wp_add_inline_style( 'slicknavcss', $slicknav_custom_css ); //Pass the variable into the main style sheet ID

}
add_action( 'wp_enqueue_scripts', 'wpb_add_inline_css' ); //Enqueue the CSS style

inline-css-wordpress2.php

<?php

//Adding CSS inline style to an existing CSS stylesheet
function wpb_add_inline_css() {
	
	wp_enqueue_style( 'slicknavcss', plugins_url() . '/slicknav/assets/css/slicknav.css' );
	
        //All the user input CSS settings as set in the plugin settings
        $slicknav_custom_css = "
            @media screen and (max-width: {$ng_slicknav_width}px) {
                {$ng_slicknav_menu} {
                  display: none;
               }
               .slicknav_menu {
                  display: block;
                  background: {$ng_slicknav_background};
               }
               .slicknav_btn {
                  background-color:{$ng_slicknav_button};
                  float:{$ng_slicknav_button_position};
               }

           }";

  //Add the above custom CSS via wp_add_inline_style
  wp_add_inline_style( 'slicknavcss', $slicknav_custom_css ); //Pass the variable into the main style sheet ID

}
add_action( 'wp_enqueue_scripts', 'wpb_add_inline_css' ); //Enqueue the CSS style