I just added a new add-ons and noticed a problem on the preview. My theme uses an SVG image in the background to make a slight gradient of color (from blue to purple).
This design is very subtle but it is unfortunately not visible on the preview.
Vous pouvez voir sur la capture le thème, et l’aperçu
Maybe I forgot something in the manifest.json? If you want to see, I wrote a tutorial (in french) and creating this theme: