If I am understanding you correctly this is very simple. You can just load your external style sheet from inside your skin template (the same exact way the template loads the default style sheet)
The top style sheet loads site wide template styles. The YOURSTYLESHEET.css loads styles for your descriptions. Keep in mind that if you use a different template for your products than you do for your home template, you'll need to link to that style sheet in your product template.
<link rel="stylesheet" type="text/css" href="skins/skin_1/style.css" />
<link rel="stylesheet" type="text/css" href="skins/skin_1/YOURSTYLESHEET.css" />
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