How to apply CSS style to all elements with same ID using one button?

Question

My idea was to create a button to hide/show all hidden text (Hide by highlighting the text in black, show by making the background color of the text transparent). But the codes,

Answer 1

Hidden text Test for more Hidden text test again. Hidden text

then the jquery:

function change() {
    var test = document.getElementById("button1");
    if (test.value == "Hide Spoiler") {
        test.value = "Open Spoiler";
        $('.p2').css('background-color','black')

    }
    else {
        test.value = "Hide Spoiler";
        $('.p2').css('background-color', 'transparent')

    }
}

I've tested it