如果输出描述时使用bloginfo(), 您可以使用过滤器bloginfo.

add_filter( \'bloginfo\', \'wpse183972_description_italic\', 10, 2 );
function wpse183972_description_italic( $text, $show )
{
    if (\'description\' == $show) {
        $word = \'WordPress\';
        $text = preg_replace( \'^(\' . preg_quote( $word ) . \')^\', \'<em>$1</em>\', $text );
    }
    return $text;
}