HTML Dropdown Looks Old on Mobile

I designed a drop-down with CSS & HTML. When I use it on a computer it looks great, but, when it's shown on mobile (either iOS and Android) the dropdown looks like it's from an older HTML. Is there a fix?

Desktop Dropdown

Mobile Dropdown

Here is the CSS for the dropdown:

.DropdownField {

  height: 45px;
  padding-left: 10px;
  width: 500px;
  font-size: 16px;
  background-color: white;

}