Beige+Orange 100% came from Anthropic intentionally training this, it's their own theme. I don't remember what their first release was that defaulted to this - it may have been Sonnet/Opus 4. Before that, not one other model used it, nor was it a particular popular theme. For light themes, white and off-white had been ultra-dominant. Anthropic themselves were the outlier with beige, which is also why the change to beige stood out so much.