WordPress not displaying information on post sent from custom admin form

Your input needs to be saved somewhere, so it can be retrieved when a user visits the website. To achieve this you could for example use the update_option() and get_option() functions from the WordPress Options API to store the values.

To store the value:

function changeStyleFunction() {
    update_option( 'newsSlide1', test_input($_POST["newsSlide1"]) );
}

To retrieve the value:

function displayNewsSlider() {
    echo get_option( 'newsSlide1', '' );
}