Add Ajax Throbber component styling

[driesd]

.ajax-progress,
.ajax-progress-throbber {
@include square(100%);
background-color: black(.5);
position: fixed;
top: 0;
left: 0;
z-index: 500;
vertical-align: middle;
text-align: center;

.message {
color: $c-white;
font-size: 20px;
padding: 30px;
}

.throbber {
@include square(128px);
float: none;
margin: 250px auto 0;
background: url("../img/misc/custom-throbber.gif") no-repeat center center;
}
}

[driesd]

Needs modification