[FEATURE] Configurable text color for dimmed/non-focused previous messages in fullscreen TUI

Problem Statement

In the fullscreen TUI renderer, previous (non-focused) user messages are rendered with the ANSI "dim"/"faint" attribute while scrolling back through history. This shows up as very low-contrast grey text that's hard to read until the message becomes the focused/current one, at which point it switches to full white text plus the theme's userMessageBackground highlight. I checked my custom theme file (~/.claude/themes/*.json) — the only overridable keys for user messages are userMessageBackground and userMessageBackgroundHover. There is no token for the dimmed/non-focused message text color, so there's no way to adjust the contrast.

Proposed Solution

Add a theme override for the dimmed/non-focused message text color (e.g. userMessageDimColor), or otherwise make this color theme-aware instead of relying purely on the ANSI dim attribute, so it can be tuned per terminal/theme instead of always rendering as very low contrast grey.

Alternative Solutions

Switching "tui" from "fullscreen" to "classic" avoids this, but gives up fullscreen-only features (flicker-free output, mouse support, auto-copy of selected text).

Priority

Low - Nice to have

Feature Category

Interactive mode (TUI)

Additional Context

Observed in a terminal multiplexer (Herdr) rendering the Claude Code pane, where ANSI dim/faint appears to further reduce contrast.