[Feature Request] Add lighter styling options for prompt area in fullscreen mode

Bug Description
I'd like to reduce the visual weight of the prompt area at the bottom of the tui in fullscreen mode. The double bars (above and below) the prompt are a bit heavy. I'd prefer a dedicated light background color or a single bar above in lighter color.

It would be even better if we could provide custom styling for all of the elements (diffs, prompt history, thoughts, etc.)

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.152
• Feedback ID: 0e13efd1-b2dd-4ac6-b6b3-a3700b3126e1

Errors

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