[FEATURE] Customizable background color for activity indicator line

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When Claude Code is working, the activity indicator line (the spinner with "Thinking...", "Reading file...", etc.) is visually similar to the rest of the terminal output. To know whether Claude is still working, you have to either consciously read the text or watch long enough to see the spinner animation move. This makes it hard to glance at the screen and instantly know the current state.

Proposed Solution

Allow users to set a background color (or other ANSI styling) on the activity indicator line via settings. For example:

{
  "activityIndicator": {
    "backgroundColor": "blue"
  }
}

Or more flexibly, allow an ANSI escape sequence so users can set any combination of foreground, background, bold, etc.:

{
  "activityIndicator": {
    "style": "\^[[44m"
  }
}

A subtle colored background (e.g. dark blue or dark green) would make the "still working" state instantly recognizable at a glance without needing to read text or watch for spinner movement.

Alternative Solutions

• The existing status line feature (statusLine in settings) is customizable but serves a different purpose (persistent info bar). The activity indicator is the transient line that only appears while Claude is actively working, which is exactly what needs to stand out.
• A more ambitious alternative would be a general theming system, but a single background color setting for the activity line would cover this use case with minimal complexity.

Priority

Medium - Nice to have improvement

Feature Category

Configuration and settings

Additional Context

Related but distinct from #23024 (customizable spinner words) and #28581 (status hook for external tooling). This request is specifically about the visual styling of the activity line to improve at-a-glance readability.