How to define a custom font family for a wordpress blog that will not affect the entire site

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You can use this function: http://codex.wordpress.org/Function_Reference/body_class

Use this in your header.php

<body <?php body_class(); ?>>

This gives each page a different class and so you can use CSS to set the font-family for a specific page like so:

body.blog span.yourClassName{ font-family:; }

Then of course the is also the jQuery solution using the .children() function