You have some custom code for Contact Form in your CSS with this header
/************************************* O.o *************************************/
/* Contact from */
/*******************************************************************************/
Which includes this:
/* -webkit-appearance: none; */
-moz-appearance: none;
appearance: none;
You can check the style.css (but If you’re not a developer, be careful, please). This affects to your input boxes, so please don’t remove.
You can custom just this input using selector and some custom code.
If you’re not comfortable with code, try a checkbox instead.