Sony’s thumbnails for each theme now includes a screenshot of the actual theme itself, rather than an icon, so you can easily see if that theme is something you would want to download. Sony has also updated the theme browser to find new themes, including the introduction of a search function. The latest version of Themes is rolling across a number of Xperia handsets right now, including Xperia XZ1, XZ Premium, older X series and Z5 models.
Thanks Ivan, Lazaros and walaspancha!