Hide Duolingo phrases, so I can practice listening in Duolingo stories

hidephrases.js

(function() {
    if (document.getElementById("zhuowei-hidephrase")) {
        document.getElementById("zhuowei-hidephrase").remove();
    }
    const elem = document.createElement("style");
    elem.id = "zhuowei-hidephrase";
    // TODO(zhuowei): autodetect the classes by going through all of the CSS
    elem.textContent = `
    ._3YBKO *, ._2rXtz .phrase * {
        color: white !important;
        -webkit-user-select: auto;
        -moz-user-select: auto;
        -ms-user-select: auto;
        user-select: auto;
    }`
    document.head.appendChild(elem);
})();