only includes css for admin

take a look at your function to write the css:

function include_css() {

		if( current_user_can( 'manage_options') ):
			echo '<style id="simpler-css-style" type="text/css">' . "\n";
			echo self::get_css();
			echo '</style><!-- end of custom css -->' . "\n";

		endif;

	}

What are you checking permissions for?

https://ww.wp.xz.cn/plugins/imporved-simpler-css/