Full-Screen Page Loading Widget

Full-Screen Page Loading Widget

Is it possible to customize the page loading widget so that
(i)  it shows a dark backdrop over the screen (like a Dialog) ?
(ii) supports a back / cancel button that returns the user if they wish to abort the load process?

For (i), would this CSS do the trick, or is there a better way ?
.ui-loader {
    padding-top: 40%;
    width: 100% !important;
    height: 100% !important;
    left: 0 !important;
    top: 0 !important;
    margin-left: 0 !important;
    margin-top: 0 !important;
}

thank you.