zen: declare dark color-scheme so menu text isn't dark-on-dark

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

templates/zen-userChrome.css

@@ -113,3 +113,13 @@
   --zen-colors-border: {{colors.outline_variant.default.hex}} !important;
   --toolbox-textcolor: {{colors.on_surface.default.hex}} !important;
 }
+
+/* Menus and dropdowns (context menus, <select>, urlbar results) take their
+   text from Gecko's system colours (-moz-MenuText, -moz-Menu), and with no
+   declared scheme the chrome computes those LIGHT — near-black text on the
+   dark background forced above, readable only on hover where the menu
+   switches to SelectedItemText. The palette generator only ever runs `dark`
+   (settings/theming.nix), so the scheme is not conditional. */
+:root, menupopup, panel {
+  color-scheme: dark !important;
+}