diff --git a/browser/themes/shared/browser-shared.css b/browser/themes/shared/browser-shared.css
index f2171eb033a1143870f4708c63f565fabb535c4b..4280bc4b0f7cdbd94179fa2111f8001a331ea42b 100644
--- a/browser/themes/shared/browser-shared.css
+++ b/browser/themes/shared/browser-shared.css
@@ -179,7 +179,6 @@ body {
   appearance: none;
 
   /* Toolbar / content area border */
-  border-bottom: 0.01px solid var(--chrome-content-separator-color);
 
   background-color: var(--toolbox-non-lwt-bgcolor);
   color: var(--toolbox-non-lwt-textcolor);
@@ -188,7 +187,6 @@ 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);
   }
 
@@ -196,6 +194,31 @@ body {
     transition: 0.8s margin-top ease-out;
   }
 
+
+  :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;
   }