Most of my Claude Code screen is English. Tool output, permission prompts, reasoning traces — paragraphs, not syntax. Standard terminal themes optimize ANSI slot colors for keywords and string literals. That's the wrong target when you're reading prose for six hours.
I built klein-blue to fix that for my own setup. Four .terminal profile variations for macOS Terminal.app, all anchored to Yves Klein's IKB pigment, APCA-verified for body-size prose legibility rather than syntax pop.
The interesting constraint: pure IKB fails as readable text on a dark ground. APCA Lc score of -12 — effectively invisible. WCAG's relative luminance ratio doesn't account for spatial frequency, so it evaluates body-size prose the same way it evaluates 24px headings and would pass a color that reads as nearly invisible at 13px. APCA Lc does model spatial frequency, which is why the per-role gates I used are size-aware: body >= 90, subtle >= 75, muted >= 45, accent >= 60.
The fix was splitting the pigment across two ANSI slots. ansi:blue gets pure IKB (hex 002FA7) for decorative work: borders, structural highlights, things you perceive rather than read. ansi:blueBright gets a lifted Klein-family blue (A8BEF0) for permission-prompt text, where legibility is the whole point.
The second design moment is ansi:redBright. Claude Code uses that slot for its claude-sand brand color. Two of the four variations neutralize it to avoid competing with Klein blue as a second hero. Two accept it. That single decision is what differentiates them visually more than anything else.
Klein Void Prot (variation 3) is the only one where every accent slot clears strict gates. The other three make deliberate tradeoffs between purity and legibility.
git clone https://github.com/robertnowell/klein-void
cd klein-void
bash install.sh
Then in Claude Code: /theme → dark-ansi. Without that step, Claude Code ignores the ANSI palette entirely and falls back to its hardcoded RGB values — the theme does nothing.
MIT licensed. Ships with CommitMono-Regular (V1, V3) and IBM Plex Mono (V2, V4), both auto-installed to ~/Library/Fonts/.
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