for the holiday event, i briefly saw some of the box had a bottom art! but...

In previous years when you clicked on a box in larger view, one of the sides was opened and you could always see the bottom plate inside this way. Not sure why that is no longer the case, but maybe the creator of said box also did not know.

You can normally only see the bottom if there is a transparent side. If you think someone is hiding something there, you can view the page source and then go to the image URL next to "data-base-bottom".

Edit: On Firefox, you can go to "tools", page info, media tab, and then scroll through the images. It won't label which image is the bottom though. I don't know if the same thing can be done on Chrome, but I'm sure there are other ways to view the image.

Edit 2: On Chrome, if you open the developer console and go to the "sources" tab, the images will be listed if you expand "imgur.com" on the left side.

I just found a neat box where all 4 sides are transparent with an animated snowing effect and there is an image on the bottom.