Stylix theme trial: swap base16Scheme, not the theming system

When a NixOS user wants to try catppuccin vs gruvbox for system-wide consistency and already uses stylix, the cheapest trial is changing stylix.base16Scheme to ${pkgs.base16-schemes}/share/themes/catppuccin-mocha.yaml on a git branch; catppuccin/nix ports are polish to add only after the palette wins. Inconsistency usually comes from apps that hardcode a theme (helix theme name, bat --theme in fzf preview, an nvim colorscheme plugin), so grep for the theme name outside stylix before blaming the palette. Also audit any rebuild script that auto-commits and pushes: unconditional 'git subtree push ... master' publishes experimental branch content to another repo's master, and 'git pull --rebase' aborts under set -e on a branch without upstream.