That tab you are hovering is "special" (because it has a protected domain addons.mozilla.org, there are a couple of them ) addons can not inject content so i assume generating a screenshot also wont work here.
Also the addon wont be able to generate previews if the tabs are discarded, so the tabs need to have been active at least once in the current sessions, otherwise there also wont be a preview available.
The download link of CSS theme doesn't work. So I created a folder chrome and pasted the text into a userChrome.css file.
But it just shows a black rectangle :(