Looks borrowed from benchmark sites and products. Themes with a light and a dark version follow your mode.
Upload a Priors theme file, or have your agent make one: copy the prompt, tell it the look you want, and upload what it returns. With the Priors skill installed it already knows the format.
Custom themes stay in this browser. They travel inside shared chart links and a page's theme, so others see them as you do.
For the builder, Explore and model pages on this device. Published pages keep the theme their owner picked.
Anthropic
Sonnet 2048 isn't ranked on General frontier yet. Best placement: #100 of 144 on LLM Chess. Results on 1 benchmarks, by reasoning effort.