feat: unify theme switching with cross-platform script

- Create unified theme.sh script that works on both WSL and macOS
- Update fish alias to use new unified script instead of WSL-specific one
- Maintain backward compatibility with existing theme command
- Update documentation to reflect unified approach
- Script automatically detects platform and calls appropriate functionality

nixos/fish.nix

@@ -4,7 +4,7 @@
   programs.fish = {
     enable = true;
     shellAliases = {
-      theme = "bash ~/dotfiles/bashscripts/wsl_theme.sh";
+      theme = "bash ~/dotfiles/scripts/theme.sh";
       music = "mpv --no-video --log-file=~/music_history.log \"$(find /mnt/c/Users/DNTH/Music/ -type f \\( -name '*.mp3' -o -name '*.wav' -o -name '*.flac' -o -name '*.m4a' -o -name '*.ogg' \\) | fzf)\"";
     };
     interactiveShellInit = ''