Adding Custom css to Alternative Stylesheet?

So I’ve edited css in the past and understand its basic functions, but still a relative newbie.

I have a theme that let me pick an alternate stylesheet – minimalist.css for our website, http://www.roundaboutmv.com.

However, whenever I try to add custom css code, it’s not picking up any of the changes.

If I get into the minimalist.css and change the code manually in a text editor and then FTP it over it works, but that process is kind of clunky.

How can I get my theme to pay attention to custom css for this alternate stylesheet? Is there something I need to add to header.php and if so, does anyone know what that code is?

Here’s my current header.php that seems to be calling the regular style.css:

<meta http-equiv=”Content-Type” content=”<?php bloginfo(‘html_type’); ?>; charset=<?php bloginfo(‘charset’); ?>” />
<link rel=”stylesheet” type=”text/css” href=”<?php bloginfo(‘stylesheet_url’); ?>” media=”screen” />
<link rel=”alternate” type=”application/rss+xml” title=”RSS 2.0″ href=”<?php if ( get_option(‘woo_feedburner_url’) <> “” ) { echo get_option(‘woo_feedburner_url’); } else { echo get_bloginfo_rss(‘rss2_url’); } ?>” />
<link rel=”pingback” href=”<?php bloginfo(‘pingback_url’); ?>” />