<p></p>
with the current code it would work by changing the colors:<br>
make the x sprite white and add a color to the background: #bbb<br>
border-radius: 10px; <br>
opacity: 0.8<br>
On hover the background would become darker, opacity go to 1.0 and the sprite could remain the same.<br>
<br>
this would render a round button and be visible also in case the banner design doesn’t take care of leaving space for the close button 

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