Transparent Background

background: rgba(255, 0, 0, 0.4);

CSS Transition

a {
  color:blue; background:white;
  -o-transition:color .2s ease-out, background 1s ease-in;
  -ms-transition:color .2s ease-out, background 1s ease-in;
  -moz-transition:color .2s ease-out, background 1s ease-in;
  -webkit-transition:color .2s ease-out, background 1s ease-in;
  /* ...and now override with proper CSS property */
  transition:color .2s ease-out, background 1s ease-in;
}
a:hover { color:red; background:yellow; }

Center Align CSS

<div id="thumbnailwrapper"> <!-- <<< This opening element -->
    <div id="artiststhumbnail">

    </div>

</div>

#artiststhumbnail {
    width:120px;
    height:108px;
    margin: 0 auto; /* <<< This line here. */
    ...
}