What an interesting question. Unfortunately I don't have an easy answer. Each time you create a password-protected post, WordPress generates a unique ID. For example, create a password-protected post then view the source code. You'll see something like the following ...

<input name="post_password" id="pwbox-20" type="password" size="20" />

Note the ID pwbox-20. Each time you create a new password-protected post another unique ID will be created. Because IDs are unique, each one has to be styled individually. Using the ID cited above you could try adding this to your style sheet ...

.protected-post-form #pwbox-20{
background:#000;
color:#fff;
}

This will give the unique "ID" pwbox-20 contained within the "class" protected-post-form a white text on black background. If you create another such post it will generate another unique ID - perhaps pwbox-21. You would adjust the style sheet as such ...

.protected-post-form #pwbox-20, .protected-post-form #pwbox-21{
background:#000;
color:#fff;
}

See the pattern? You would have to keep modifying the style sheet.

In CSS classes are defined by a . while IDs are defined by a #. IDs are unique while classes are not. Here is a good explanation of the differences between the two.

Maybe there is an easier way but I can't think of it.

It all worked fine but i am still unable to change the input txt from white to black or change the bg color (the actual area where you'd write the password).

Any help?

peace