Letter in contact form are "invisible"

Hello,

I am an amatuer web designer trying to make for a friend’s website a wordpress redesign.

I currently have the problem, that the contact form doesn’t show the written letters. They are there but the letters are “invisible”. I know that I can find the solution in the style.css and change the letter color or so but I am searching and changing for about three hours now any “color values” that I can find and still the problem didn’t disappear. So please be my last hope and say that you have any suggestions on where I’ve misunderstood the problem.

You can see the described problem here: https://www.thestatesman.info/contact/

I hope that you know the solution. Thank you.