Newsletter block - Change colour of the 'First Name' and 'Email Address' default grey colour

Hello,

I would need your help on some customisation for my Newsletter block.

Here's the situation, I've added some CSS code to have a one line at the bottom field box.

I would like to make the 'First Name' and 'Email Address' text in white (instead of the default grey) and remove the thick white outline when I fill in the box with my First Name and also Email Address. (Images Attached)

I've also attached the CSS code I've already used on the newsletter block.

Thanks,

Roxane