Preview on add-ons does not display SVG images?

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:

Thank you

On another note, they don’t display in the review queue either. I just assumed that I was seeing what it was supposed to look like in the previews. Guess not.

You can file an issue to add SVG support here.