tabs below content: Use toolbox background color/image for tabstoolbar

This makes tabs toolbar use background-image(s) from the selected theme
and should make sure the background-color is correctly set which should
fix #381

chrome/tabs_below_content.css

@@ -8,7 +8,6 @@ Window controls will be all wrong without it.
 Additionally on Linux, you may need to get:
 linux_gtk_window_control_patch.css
 */
-/* Note: You must restart Firefox if you change the setting to select if firefox should hide toolbars in fullscreen. */
 
 .titlebar-buttonbox-container{
   position: fixed;
@@ -59,10 +58,22 @@ linux_gtk_window_control_patch.css
   width: 100vw;
   height: calc(var(--tab-min-height) + 2 * var(--tab-block-margin,0px));
   --tabs-navbar-shadow-size: 0px;
+  background-color: light-dark(rgb(240, 240, 244), rgb(28, 27, 34)) !important;
+  z-index: 1;
+  :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) !important;
+  }
+  :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);
+  }
 }
 #TabsToolbar:hover{ z-index: 1 }
 
-:root[lwtheme] #TabsToolbar{ background-color: var(--lwt-accent-color); }
 #TabsToolbar-customization-target{ width: 100vw; }
 
 .tab-background{ border-top-style: none !important; }