Animated gifs are intended for sharing off the site. This creates a slight opportunity for .com to market itself. They've chosen the current theme you're looking at because it's elegant and professional.
Animated GIFs


Also, it would take a lot of work to render different themes in a gif, and it's not like a gif affects the way you play as with the live chess sets... and it's not as if anybody is actually going to analyse the gif... so it just makes sense to load the default tournament theme.
Hey, I just wanted to say kudos for the animated GIFs function. I just wish you could change the piece graphics on it in settings somewhere.
And if you can, well then that's another blunder I've made.