How to add inline styles the right way

inline-styles.php

<?php
/*
* As a wise theme reviewer advised - theme options are required to be within the Customizer!
*/
function themename_custom_css_adjust() {
	$css      = '';
	
	$bg_color = get_theme_mod( 'themename_menu_bg_color' );
	$link_color = get_theme_mod( 'themename_last_link_color' );
	$link_hover = get_theme_mod( 'themename_last_link_hover' );
	
	$css .= '
		.main-navigation li:last-child {background-color: ' . $bg_color . ';}
		.main-navigation li:last-child a {color:  ' . $link_color . ';}
		.main-navigation li:last-child a:hover {color:  ' . $link_hover . ';}
	';
	}
	wp_add_inline_style( 'themename-style', $css ); // Assuming that `theme-style` is the handle for your main stylesheet enqueue.
}
add_action( 'wp_enqueue_scripts', 'themename_custom_css_adjust' );