diff --git a/browser/themes/shared/browser-shared.css b/browser/themes/shared/browser-shared.css
index 08bc078c2572fb218b435e580ef71be2586989c3..e440b6dc3f331785c841e1702887f67a67e00e96 100644
--- a/browser/themes/shared/browser-shared.css
+++ b/browser/themes/shared/browser-shared.css
@@ -144,7 +144,6 @@ body {
   transition: background-color var(--inactive-window-transition);
 
   &:-moz-window-inactive {
-    background-color: var(--toolbox-non-lwt-bgcolor-inactive);
     color: var(--toolbox-non-lwt-textcolor-inactive);
   }
 
@@ -163,6 +162,31 @@ body {
     border-bottom-color: var(--chrome-content-separator-color);
   }
 
+
+  :root[customizing] & {
+    border-bottom-style: none;
+  }
+
+  :root[lwtheme] & {
+    background-image: var(--lwt-additional-images);
+    background-repeat: var(--lwt-background-tiling);
+    background-position: var(--lwt-background-alignment);
+    background-color: var(--lwt-accent-color);
+    color: inherit;
+
+    &:-moz-window-inactive {
+      background-color: var(--lwt-accent-color-inactive, var(--lwt-accent-color));
+    }
+  }
+
+  /* When a theme defines both theme_frame and additional_backgrounds, show
+     the latter atop the former. */
+  :root[lwtheme-image] & {
+    background-image: var(--lwt-header-image), var(--lwt-additional-images);
+    background-repeat: no-repeat, var(--lwt-background-tiling);
+    background-position: right top, var(--lwt-background-alignment);
+  }
+
   :root[customizing] & {
     border-bottom-style: none;
   }