feat(theme): introduce retro Windows 98-inspired theme

Add a new "retro" theme option that mimics the classic Windows 98 aesthetic, providing a nostalgic alternative to the modern and classic dark themes.

Changes include:
- Defining CSS variables for the retro theme in `00-base.css` (pixel fonts, silver/gray colors, and classic window shadows).
- Adding custom styling for cards, headers, buttons, and title bars to replicate classic OS windows.
- Adding a star background GIF (`stars1.gif`).
- Excluding the retro theme from modern "revamp" styles in `15-revamp.css`.
- Updating `CLAUDE.md` with instructions on screenshot verification.

CLAUDE.md

@@ -2,6 +2,9 @@
 
 This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
 
+## Important Nots
+Do not take screenshots yourself, ask the user to take screenshots of your visual changes if you want to so that you can verify them.
+
 ## Commands
 
 **Go Executable:**