Add Caishen Lite Readable theme

[ChannelerH]

Summary

• Adds Caishen Lite Readable to prompts/themes.json.
• Adds the theme to the README showcase table.
• Uses a public HTTPS CDN asset, without committing image binaries.

Theme details

• Name: Caishen Lite Readable / 财神浅色可读版
• Appearance: light
• Image size: 2172 x 724
• Showcase URL: https://codex-theme-gallery.howardhua.chatgpt.site/contrib/aithink001/caishen-lite-readable-banner.png
• WebP preview URL: https://codex-theme-gallery.howardhua.chatgpt.site/contrib/aithink001/caishen-lite-readable-banner.webp
• Source/theme page: https://codex-theme-gallery.howardhua.chatgpt.site/themes/caishen-lite

Rights and method

• Public-safe original artwork prepared for Codex theme use.
• No private Codex workspace screenshots, fake app chrome, logos, watermark, or protected character assets.
• Cropped/exported as a 3:1 banner with low-detail space on the left and the main subject on the right.

Validation

• Parsed prompts/themes.json locally.
• Verified the PNG CDN URL returns HTTP 200.
• Confirmed no image binaries are committed to the repo.