How do I save the option chosen by the html user in cookie? [duplicate]

I suggest using the code on the MDN page to create and read cookies. (I also put the bottom of the answer.)

And in this case the syntax will be:

  docCookies.getItem (name, value [ end [ path [ domain [ secure]]]) - create the cookie
docCookies.getItem (name) - read the cookie
docCookies.removeItem (name [ path [ domain]]) - remove the cookie

Using this code as support, you can then use this:

var fundo = docCookies.getItem('corDeFundo') || "#fff";
$('body').css('background-color', fundo);
$('button').on('click', function(){
    fundo = $(this).data('value');
    docCookies.setItem('corDeFundo', fundo);
    $('body').css('background-color', fundo);
});

Example: link

(Test refresh the example after you have changed the color and you will see that Cookie stores the information.

Code at MDN page :

var docCookies = {
  getItem: function (sKey) {
    if (!sKey) { return null; }
    return decodeURIComponent(document.cookie.replace(new RegExp("(?:(?:^|.*;)\s*" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\$&") + "\s*\=\s*([^;]*).*$)|^.*$"), "$1")) || null;
  },
  setItem: function (sKey, sValue, vEnd, sPath, sDomain, bSecure) {
    if (!sKey || /^(?:expires|max\-age|path|domain|secure)$/i.test(sKey)) { return false; }
    var sExpires = "";
    if (vEnd) {
      switch (vEnd.constructor) {
        case Number:
          sExpires = vEnd === Infinity ? "; expires=Fri, 31 Dec 9999 23:59:59 GMT" : "; max-age=" + vEnd;
          break;
        case String:
          sExpires = "; expires=" + vEnd;
          break;
        case Date:
          sExpires = "; expires=" + vEnd.toUTCString();
          break;
      }
    }
    document.cookie = encodeURIComponent(sKey) + "=" + encodeURIComponent(sValue) + sExpires + (sDomain ? "; domain=" + sDomain : "") + (sPath ? "; path=" + sPath : "") + (bSecure ? "; secure" : "");
    return true;
  },
  removeItem: function (sKey, sPath, sDomain) {
    if (!this.hasItem(sKey)) { return false; }
    document.cookie = encodeURIComponent(sKey) + "=; expires=Thu, 01 Jan 1970 00:00:00 GMT" + (sDomain ? "; domain=" + sDomain : "") + (sPath ? "; path=" + sPath : "");
    return true;
  },
  hasItem: function (sKey) {
    if (!sKey) { return false; }
    return (new RegExp("(?:^|;\s*)" + encodeURIComponent(sKey).replace(/[\-\.\+\*]/g, "\$&") + "\s*\=")).test(document.cookie);
  },
  keys: function () {
    var aKeys = document.cookie.replace(/((?:^|\s*;)[^\=]+)(?=;|$)|^\s*|\s*(?:\=[^;]*)?(?:|$)/g, "").split(/\s*(?:\=[^;]*)?;\s*/);
    for (var nLen = aKeys.length, nIdx = 0; nIdx < nLen; nIdx++) { aKeys[nIdx] = decodeURIComponent(aKeys[nIdx]); }
    return aKeys;
  }
};

You can use cookies, as already shown in the other response, or use local storage, which is very simple and does not load your requests with unnecessary data.

Here an excellent post on the subject.