[Feature Request] Add theme tokens for code block background styling

Bug Description
▎ Feature request: theme tokens for code block styling. On light themes, fenced code blocks in responses are hard to distinguish from prose because they render as foreground-only syntax colors with no background fill. Please add a codeBlockBackground token (and ideally a paired foreground/text token per region) to the custom theme system, similar to how userMessageBackground works in fullscreen mode. Diff rendering with its background fill is very readable — code blocks deserve the same treatment.

Environment Info

• Platform: darwin
• Terminal: WarpTerminal
• Version: 2.1.234
• Feedback ID: b50f744a-17e0-4632-9ae6-52bc766fca7d

Errors

[{"error":"AxiosError: Request failed with status code 400\n    at s8e (/$bunfs/root/cli:108:1199)\n    at <anonymous> (/$bunfs/root/cli:113:15342)\n    at emit (node:events:97:22)\n    at endReadableNT (internal:streams/readable:901:50)\n    at processTicksAndRejections (native:7:39)\n    at request (/$bunfs/root/cli:115:2513)\n    at async Qrd (/$bunfs/root/cli:313:1965)\n    at async jkh (/$bunfs/root/cli:23078:54451)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-08-18T08:00:26.362Z"}]