Dim assistant prose body text in dark mode
The prose body used gray-300 (#d4d4d4) in dark mode, which read slightly
too bright. Gray-400 (#a3a3a3) is too dim a jump, so mix the two for a
half-step (~#bcbcbc).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nt5HnqeFaD7cd7XhzgW55
