Three hours into a Claude Code session I noticed my eyes were doing more work on the permission prompts and reasoning blocks than on any code. Those are English sentences at body size, not identifiers on a dark ground — and every terminal theme I tried was tuned for the latter.
The specific problem with IKB, the pigment Yves Klein called International Klein Blue (hex 002FA7): it measures Lc -12 on APCA against a dark background. Effectively invisible as readable text. I still wanted it in the theme, so it lives in ansi:blue as a decorative slot — borders, highlights — while a lifted Klein-family blue (A8BEF0) carries the permission-prompt text in ansi:blueBright where it actually needs to pass Lc 60+.
Klein-blue ships four variations, each making a different trade-off on the ansi:redBright slot, which Claude Code uses for its claude-sand brand color:
- Klein Void Refined — neutralizes claude-sand so IKB is the single anchor
- Klein Void Sand & Sea — accepts claude-sand as a second hero alongside IKB
- Klein Void Prot — every accent slot APCA-verified; body text gates at Lc 90, subtle at Lc 75
- Klein Void Gallery — one-blue maximum void, minimal palette
The contrast model uses per-role APCA gates throughout: body >= 90, subtle >= 75, muted >= 45, accent >= 60. V3 Prot is the only variation where every slot clears strict gates; the others make intentional aesthetic trades below that ceiling.
One setup requirement worth knowing up front: you have to set Claude Code's /theme picker to dark-ansi. Without it, Claude Code ignores the Terminal.app ANSI profile entirely and falls back to its hardcoded RGB palette.
Ships as .terminal profile files for macOS Terminal.app. Built from build.m with a variation-aware Objective-C builder; install.sh and restore.sh handle setup and rollback.
git clone https://github.com/robertnowell/klein-blue
cd klein-blue
bash install.sh
github.com/robertnowell/klein-blue
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