Customizing Sprout Invoice Templates (via CSS)

functions.php

<?php
/**
 * Hook into si_head hook and add a new custom CSS stylesheet.
 * 
 */
function si_add_stylesheet() {
	echo '<link rel="stylesheet" id="sprout_doc_style-css" href="'.get_stylesheet_directory_uri() . '/css/sprout-invoices.custom-style.css" type="text/css" media="all">';
}
add_action( 'si_head', 'si_add_stylesheet' );

// OR

/**
 * Hook into si_head hook and add custom CSS.
 * 
 */
function si_add_css() {
	?>
		<style type="text/css">
			#logo img {
				margin-top: -50px;
				max-height: 180px;
			}
		</style>
	<?php
}
add_action( 'si_head', 'si_add_css' );