Yves Klein registered International Klein Blue as a pigment in 1960. Pure IKB (hex 002FA7) on a dark terminal ground measures Lc -12 on the APCA contrast scale — effectively invisible as text. That's the central problem I had to solve when building klein-blue.
Claude Code uses specific ANSI slots for specific jobs: ansi:blue for decorative borders, ansi:blueBright for permission-prompt text you actually need to read. So I split IKB across both slots. Pure 002FA7 goes into the decorative slot where legibility doesn't matter. A lifted Klein-family blue (A8BEF0) goes into the readable slot where APCA body contrast gates apply.
The four variations each handle this trade-off differently. Klein Void Prot is the strictest: every accent passes APCA gates (body >= 90, subtle >= 75, muted >= 45, accent >= 60). Klein Void Gallery pushes maximum void — one blue, minimal interruption. Klein Void Sand & Sea accepts Claude's ansi:redBright slot as a second hero color instead of neutralizing it. Klein Void Refined splits the difference.
All of them are tuned for prose, not syntax. Claude Code is paragraphs of tool output, reasoning, and permission prompts for hours at a stretch. Standard themes optimize for the 20% that's code.
Ships as macOS Terminal.app .terminal profiles with CommitMono-Regular (V1, V3) and IBM Plex Mono (V2, V4) — both install to ~/Library/Fonts/ automatically. One thing that isn't obvious: Claude Code has to be set to dark-ansi via /theme, otherwise it ignores the ANSI profile entirely and uses its hardcoded RGB palette.
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