That particular piece of text is not specifically styled. You could edit

"./includes/templates/template_default/sideboxes/tpl_mailchimp_sidebox.php"

and change

$content .= BOX_MAILCHIMP_PITCH;

to

$content .= '<span id="foo">' . BOX_MAILCHIMP_PITCH . "</span>";

and add

.foo {
styling
}

to your css file.