diff --git a/settings/theming.nix b/settings/theming.nix
index 65f64dd..ff8dd52 100644
--- a/settings/theming.nix
+++ b/settings/theming.nix
@@ -177,7 +177,12 @@ let
 
       headerbar { min-height: 0; padding: 0; margin: 0; }
       headerbar .title { font-size: 0; }
-      window:backdrop {
+      /* .background, not bare `window`: GTK puts that class on a window that
+         paints a solid background, and a transparent app takes it off — ghostty
+         drops it whenever background-opacity < 1. Unscoped, this rule painted
+         @window_bg_color back over ghostty the moment it lost focus, so a 70%
+         terminal went fully opaque on every alt-tab. */
+      window.background:backdrop {
         background-color: @window_bg_color;
         color: @window_fg_color;
       }