wp-comments-post.php – how to change font-size via CSS?

Hi guys,
When testing the comments form on blog pages and hitting the submit button without filling out all required fields, an error is shown on a wp-comments-post.php page reminding the user to fill out the missing fields.
The font-size there is too small for my needs but I haven't been able so far to change it via CSS.
The source code says

#error-page p {
	font-size: 14px;
	line-height: 1.5;
	margin: 25px 0 20px;
}

but I can't manage to overwrite that.
Even

#error-page p {
	font-size: 18px !important;
}

has no effect. Why that?
Thanks for any help!