[libadwaita/wip/exalm/background] stylesheet: Support .navigation-sidebar.background

[Alexander Mikhaylenko <alexm src gnome org>]

commit fa625959ba5deec77aab274d2822a1abda1f1a1c
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Thu Nov 4 13:55:59 2021 +0500

    stylesheet: Support .navigation-sidebar.background
    
    When removing background from .navigation-sidebar I forgot flaps need
    background. Let's support this.

 src/stylesheet/widgets/_sidebars.scss | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/src/stylesheet/widgets/_sidebars.scss b/src/stylesheet/widgets/_sidebars.scss
index 9225da8c..cb9f0f37 100644
--- a/src/stylesheet/widgets/_sidebars.scss
+++ b/src/stylesheet/widgets/_sidebars.scss
@@ -53,6 +53,11 @@ separator.sidebar {
     color: inherit;
   }
 
+  &.background {
+    background-color: $window_bg_color;
+    color: $window_fg_color;
+  }
+
   padding: $menu-margin 0; //only vertical padding. horizontal row size would clip
 
   > separator {