Style wordpress protected posts

I assume you mean styling the password protected posts on the front-end.

a) CSS:

If your theme is using the post_class() function, like:

<article <?php post_class(); ?>>...</article>

then it will generate:

<article class="... post-password-required ...">...</article>

for the password protected posts.

So you can simply target these posts with:

.post-password-required {    background-color: #eee;}

in your stylesheet.

b) Form:

i) If you want to add text to your password form, you can use the following:

add_action( 'the_password_form', 'rob_the_password_form' );function rob_the_password_form( $output ){    $before = ' Before ';  // Modify this to your needs!    $after  = ' After ';   // Modify this to your needs!    return $before . $output . $after;}

ii) If you want to modify the HTML form directly, you can override it with:

add_filter( 'the_password_form', 'rob_override_the_password_form' );function rob_override_the_password_form( $form = '' ) {    global $post;    $label = 'pwbox-'.( empty( $post->ID ) ? rand() : $post->ID );    $form = '<form action="' . esc_url( site_url( 'wp-login.php?action=postpass', 'login_post' ) ) . '" method="post">    ' . __( "To view this protected post, enter the password below:" ) . '    <label for="' . $label . '">' . __( "Password:" ) . ' </label><input name="post_password" id="' . $label . '" type="password" size="20" maxlength="20" /><input type="submit" name="Submit" value="' . esc_attr__( "Submit" ) . '" />    </form>    ';    return $form;}

where this is based on the example from the Codex. It uses the same form as the default one.

c) Title:

To change the default Protected: title prefix, you can use:

add_filter( 'protected_title_format', 'rob_protected_title_format' );function rob_protected_title_format( $format ){    $format = __( ' Members only! %s ' ); // Modify this to your needs!    return $format;}

Hopefully this will help you to style your protected posts.

You can use Protected Post Personalizer plugin for doing the trick https://wordpress.org/plugins/protected-post-personalizer/