Well, looks like the old CSS doesn’t work for desktop Topics anymore, but here’s an update:
$url_of_christmas_hat_asset:'###my-hat-url###';
.post-avatar::after {
position: absolute;
content: '';
display: block;
top: -30px;
left: -24px;
width: 80px;
height: 80px;
background: url($url_of_christmas_hat_asset) no-repeat center;
background-size: contain;
}
Which is nice, because this is now generic for Desktop & Mobile
But you probably want to use one of the packaged Theme Components …