Hi Edwin.
Custom CSS is needed to change colors there :
select::placeholder, textarea::placeholder, input[type="text"]::placeholder, input[type="password"]::placeholder, input[type="datetime"]::placeholder, input[type="datetime-local"]::placeholder, input[type="date"]::placeholder, input[type="month"]::placeholder, input[type="time"]::placeholder, input[type="week"]::placeholder, input[type="number"]::placeholder, input[type="email"]::placeholder, input[type="url"]::placeholder, input[type="search"]::placeholder, input[type="tel"]::placeholder, input[type="color"]::placeholder, .uneditable-input::placeholder {color: #ccc;}
.sppb-form-control, .sppb-form-group .sppb-form-control {color: #eee;}
I added here: templates/shaper_revibe/css/custom.css (and done, contrast improved)