Allow theming the named-session (claude -n) display-name chip background color

What

When launching a named session with claude -n "Some Name", the display-name chip rendered on the top border of the prompt input box has a hardcoded cyan (ansi:cyan) background. There appears to be no supported way to change it.

Why this is a problem

Users who run a custom theme (~/.claude/themes/*.json) to get a neutral / non-cyan look can override promptBorder, promptBorderShimmer, background, etc., but none of these affect the named-session chip - it stays cyan. Specifically:

• Overriding the background theme token to grey leaves the chip cyan (so it doesn't read that token).
• No CLI flag, settings.json entry, or environment variable sets the chip color.
• /color sets only the prompt border color, and its options (red/blue/green/yellow/purple/orange/pink/cyan) include no neutral/grey.

So for anyone running a deliberately neutral/monochrome theme, the bright cyan chip is the one element that can't be brought in line.

Request

Expose the named-session chip background as either (or both):

1. A custom-theme token (e.g. sessionNameChip / sessionChipBackground), overridable in ~/.claude/themes/*.json.
2. A /color target (and add a neutral/grey option).

Environment

• Claude Code 2.1.183
• macOS, iTerm2

Notes

Verified against the 2.1.183 binary: no #00CCCC/rgb(0,204,204) hex exists, while ansi:cyan is a hardcoded literal - consistent with the chip color not being a themeable token.