[libadwaita/wip/exalm/borderless: 37/46] Revert "Revert "stylesheet: Restore a backdrop state for header bar""

[Alexander Mikhaylenko <alexm src gnome org>]

commit 6be9c12761b9830bca7eb6687d24b6d2d28c6690
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Tue Jun 29 13:20:05 2021 +0500

    Revert "Revert "stylesheet: Restore a backdrop state for header bar""
    
    This reverts commit 284af49bcc883959609eb3729f7eb4a0d74ca73d.

 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 0a0c4c99..193fdf41 100644
--- a/src/stylesheet/widgets/_header-bar.scss
+++ b/src/stylesheet/widgets/_header-bar.scss
@@ -19,15 +19,17 @@ 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;
     }
   }