.PromptBoxContainer {
    height: 100%;
    width: 100%;
    position: fixed;
    top: 0;
    left: 0;
    z-index: 1000;
    background-color: rgba(0,0,0,0.5);
}

    .PromptBoxContainer .PromptBoxDisplay {
        background-color: white;
        position: fixed;
        width: 500px;
        height: 260px;
        top: 50%;
        left: 50%;
        margin-top: -130px; /* Negative half of height. */
        margin-left: -250px; /* Negative half of width. */
    }