[Bug] User message background overrides terminal transparency

Bug Description
Title: User message background overrides terminal transparency

Body:

When using Claude Code in a terminal with background transparency (e.g. Yakuake/Konsole, Alacritty), the user's messages in the conversation history are rendered with an explicit ANSI background color. This creates opaque blocks that break the terminal's transparency effect.

Only the user messages are affected — the rest of the UI (assistant responses, tool output) respects the default terminal background and appears transparent as expected.

Expected behavior: User messages should use the terminal's default background (no explicit ANSI bg color), so terminal transparency applies uniformly.

Environment: Yakuake (Konsole backend), Alacritty, Linux

Environment Info

• Platform: linux
• Terminal: konsole
• Version: 2.1.86
• Feedback ID: 18c0e752-2d03-45a8-8df4-27bd00e77cb3

Errors

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