Custom Stylesheet

I am using a theme that gets periodic updates. I also have made several modifications to the theme’s stylesheet. My problem is that on every theme update, I must go back into the stylesheet and re-modify the css.

I am therefore, trying to generate a modification-only stylesheet that my theme will recognize and will use the modified code over the standard stylesheet.

Below is the theme’s stylesheet section in the header, with my modifications stylesheet as the last bit of code (scotts.css).

I cannot get this to work…any ideas?

<!--
Stylesheet
~~~ -->

<?php if (function_exists('hmt_extras')) { 				// Extras plugin? Then go get the css ?>
	<link rel="stylesheet" href="<?php bloginfo('wpurl'); ?>/wp-content/plugins/heatmap-extras/menu.css" type="text/css" media="screen" />
<?php } ?>

<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen" />

<?php if (!function_exists('hmt_child_theme')) { 				// If there is no child theme then grab the default.css instead ?>
	<link rel="stylesheet" href="<?php bloginfo('template_url'); ?>/default.css" type="text/css" media="screen" />
<?php } ?>

<?php $stylesheetfile = STYLESHEETPATH . '/css/scotts.css'; ?>
<link rel="stylesheet" href="<?php echo $stylesheetfile; ?>" type="text/css" media="screen" />

-Scott