[FEATURE]: Custom Theme / Color Configuration [for Transparent Terminal Support]

Use Case

I want to use Claude Code with a fully transparent terminal (Warp at 0% opacity) so I can watch videos/content behind the terminal while working. Currently, Claude Code's hardcoded background colors make this unusable.

Requested Features

1. Configurable diff view colors

• Option to remove/disable background highlighting on diffs
• Or set diff backgrounds to transparent

2. Message opacity by age

• Reduce opacity of messages older than the most recent N messages
• Helps focus on current context while older messages fade

3. Input box styling

• Option for slight dark opacity/tint on input area
• Makes input readable over transparent background

4. General theme customization

• Expose color values in settings.json or ~/.claude.json
• Allow per-element color overrides (diff bg, message bg, input bg, etc.)

Why this matters

Transparent terminals are popular for:

• Aesthetic preferences
• Screen real estate efficiency
• Watching videos/TV while coding
• Reference material visible behind terminal

Current /theme command only offers preset themes with no granular control.

Suggested implementation

{
  "theme": {
    "base": "dark",
    "overrides": {
      "diffAddBackground": "transparent",
      "diffRemoveBackground": "transparent",
      "messageBackground": "rgba(0,0,0,0.1)",
      "inputBackground": "rgba(0,0,0,0.3)",
      "olderMessageOpacity": 0.6
    }
  }
}