button,
.button,
.vc_btn3,
.wpforms-submit,
.subscribe_button,
input[type="submit"] {
  box-shadow: none !important;
  text-shadow: none !important;
}
To target one button, right-click it, choose Inspect, and send me the class name. I can then give you a precise selector. If you also see a shadow on hover or click, add :hover and :focus versions of the selector.

give me custom css to make buttons flat-colored

Here’s flat-colored button CSS using your navy and white. It removes shadows, gradients, and 3D effects.

css
button,
.button,
.vc_btn3,
.wpforms-submit,
.subscribe_button,
input[type="submit"] {
  background: #123F63 !important;
  background-image: none !important;
  color: #ffffff !important;
  border: 0 !important;
  box-shadow: none !important;
  text-shadow: none !important;
  text-decoration: none !important;
  transition: background 0.2s ease, color 0.2s ease;
}

button:hover,
button:focus,
.button:hover,
.button:focus,
.vc_btn3:hover,
.vc_btn3:focus,
.wpforms-submit:hover,
.wpforms-submit:focus,
.subscribe_button:hover,
.subscribe_button:focus,
input[type="submit"]:hover,
input[type="submit"]:focus {
  background: #2F8251!important;
  color: #ffffff !important;
  box-shadow: none !important;
}