Using the xoosla theme, the file containing the icon (favicon) is /themes/xoosla/xotpl/xohead.html. Here's the confusing part. In the file there are two lines:

<link rel="shortcut icon" type="image/ico" href="<{xoImgUrl icons/favicon.ico}>" />
<link rel="icon" type="image/png" href="<{xoImgUrl icons/icon.png}>" />

Normally, we would think that the "favicon.ico" line is the one showing the favicon. It's not! It's the one right underneath that, the "icon.png" line.