That is displaying incorrectly. If no screenshot is added to the theme, it should show an SVG generated from the theme’s colour palette. It should also show the theme’s name below that image. That’s what is shown in the announcement here.
Looking at this previous topic, it looks like you might have added some custom CSS for that page. Perhaps that is interfering here? It also looks like your layout is incorrect as those theme cards are displaying too widely and wrapping incorrectly.
Could you check if you have some CSS affecting this page?