Desktop app: highlight color for open sessions in the sidebar is too subtle and not customizable

Problem

In the Claude Code desktop app (macOS, light mode), sessions that are currently open are indicated with a very light gray background in the "Recents" section of the sidebar. The contrast against the normal background is so low that it is genuinely hard to tell which sessions are highlighted. There is currently no theme token or setting to change this color — the custom theme system (~/.claude/themes/) does not expose sidebar highlight colors.

Proposed solution (any of these would help)

1. Expose a theme token for the sidebar "open session" highlight so users can choose a more visible color (e.g., pale yellow / orange / green).
2. Increase the default contrast of the highlight, or add a stronger visual cue (colored left border or dot) instead of relying only on a subtle background tint.
3. Respect the OS-level "Increase contrast" accessibility setting.

Environment

• Claude Code desktop app on macOS (Darwin 25.4.0)
• Light mode, Japanese UI