Add Pi theme based on system theme

Based entirely on the work of @robzolkos in #5711. That attempt was
adapted to better fit the standard templating system while also adding
color mixing for use in other themes.

bin/omarchy-theme-set

@@ -173,6 +173,7 @@ post_theme_commands=(
   omarchy-restart-helix
   omarchy-theme-set-foot
   omarchy-theme-set-gnome
+  omarchy-theme-set-pi
   omarchy-theme-set-browser
   omarchy-theme-set-vscode
   omarchy-theme-set-obsidian