To change the colors for stripe form you need to edit in code the following colors.

File path:
wprentals/js/wpestate-stripe.js

At line 17 you have


var card = elements.create('card', {
iconStyle: 'solid',
style: {
base: {
iconColor: '#c4f0ff',
color: '#fff',
fontWeight: 500,
fontFamily: 'Roboto, Open Sans, Segoe UI, sans-serif',
fontSize: '16px',
fontSmoothing: 'antialiased',

':-webkit-autofill': {
color: '#fce883',
},
'::placeholder': {
color: '#87BBFD',
},
},
invalid: {
iconColor: '#FFC7EE',
color: '#FFC7EE',
},


Change the value of the colors in there.