Why I split a single color across two ANSI slots

I spent several weeks reading Claude Code output for hours at a stretch and the default themes were wearing me out. Not because the syntax highlighting was wrong — Claude Code is mostly prose. Tool output, reasoning traces, permission prompts: paragraphs, not tokens. The themes I tried were tuned for code; nobody had tuned one for body-size English over a long session. So I built klein-void, four Terminal.app themes built around Yves Klein's International Klein Blue. The design problem that ate most of the time was figuring out why pure IKB kept failing as readable text even when it looked fine to my eye. WCAG contrast ratios don't model how the visual system perceives lightness on dark backgrounds. WCAG uses a symmetric ratio: the calculation is identical regardless of polarity. APCA (Accessible Perceptual Contrast Algorithm) is asymmetric — dark-on-light and light-on-dark are scored differently because the eye doesn't treat them the same. Pure IKB on a dark ground scores Lc -12