In your templates' index.php
you can just reference the common assets:
<head>
<jdoc:include type="head" />
<link rel="stylesheet" href="<?php echo $this->baseurl ?>templates/common/css/style.css" type="text/css" />
<link rel="stylesheet" href="<?php echo $this->baseurl ?>templates/this_template/css/style.css" type="text/css" />
</head>
Then, using Template styles, you can assign the templates to menu items.