Opus 4.7 selects such palette and motifs by default. Might even be first iteration of claude design.

This design still screams Claude to me, but a newer version than what you're thinking of. At some point they added a markdown file that tells it to use obviously AI designs like lots of blue/purple and gradients. Since then, this is its new style.

DeepSeek v4 perhaps?

Frontend design skill by Anthropic specifically says not to use purple. I'd be surprised if it still uses purple. Have you seen that recently?