As the designer of the site you can control what css is applied to the form fields initially or when certain actions happen with the use of tools like the spry validations.
If you have a browser add in that fills the forms for you and recolors the input fields then this does not have anything to do with your site or the css you are using, it is an effect of the add in you are using. You should check the options you have for the add in to see if this is something you can change or disable.