You can blank the $errorCss static property of CHtml in your view like so:
<?php CHtml::$errorCss = ""; ?>
However, this setting will be global to your entire application.
If you want to change the styling of error for any particular label or page, override it in your CSS based on the context. For example, if your labels or input fields are within a box with an id of "login," you could override the styling like so: