[libadwaita/wip/exalm/headerbars: 2/4] stylesheet: Restore a backdrop state for header bar

[Alexander Mikhaylenko <alexm src gnome org>]

commit c26231eb1cdb0e290a9980d96c15cc8f5da0cb4d
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Wed Jun 16 17:06:08 2021 +0500

    stylesheet: Restore a backdrop state for header bar
    
    Use a slight brightness filter, make backdrop header bars slightly darker
    than the active one.

 src/stylesheet/widgets/_header-bar.scss | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/stylesheet/widgets/_header-bar.scss b/src/stylesheet/widgets/_header-bar.scss
index 78b4435a..f7f16f67 100644
--- a/src/stylesheet/widgets/_header-bar.scss
+++ b/src/stylesheet/widgets/_header-bar.scss
@@ -16,15 +16,17 @@ headerbar {
   }
 
   &:backdrop {
-    background-color: $headerbar_bg_color;
+    filter: brightness(.92);
 
     transition: $backdrop_transition;
+    transition-property: filter;
 
     > windowhandle {
       // opacity looks weird with GtkSwitch, but filter works fine
       filter: opacity(0.5);
 
       transition: $backdrop_transition;
+      transition-property: filter;
     }
   }