Email Form 'Placeholder' text styling?

Hi - I’ve tried to track down the bootstrap 4 class for the ‘placeholder’ text inside form-input field, as I want to change the color and text style…I even used the ‘Inspector’ to find it, but no luck.

Hope someone can guide me with this!

I also tried placing this code in the ‘Header’ code box for the page:

::placeholder {
  color: white;
  opacity: 1;