add some backdrop transitions

common/gtk-3.0/3.20/gtk-dark.css

@@ -896,6 +896,7 @@ headerbar,
     border-color: rgba(38, 42, 51, 0.97); }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(207, 218, 231, 0.5);
     background-color: #323843; }
     .csd headerbar:backdrop,
@@ -2858,7 +2859,8 @@ decoration {
   box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.97), 0 8px 8px 0 rgba(0, 0, 0, 0.35);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.87), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.35); }
+    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.87), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.35);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/gtk-darker.css

@@ -895,6 +895,7 @@ headerbar,
     border-color: rgba(38, 42, 51, 0.97); }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(207, 218, 231, 0.5);
     background-color: #323843; }
     .csd headerbar:backdrop,
@@ -2857,7 +2858,8 @@ decoration {
   box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.97), 0 8px 8px 0 rgba(0, 0, 0, 0.2);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.87), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2); }
+    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.87), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/gtk-solid-dark.css

@@ -896,6 +896,7 @@ headerbar,
     border-color: #262a33; }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(207, 218, 231, 0.5);
     background-color: #323843; }
     .csd headerbar:backdrop,
@@ -2858,7 +2859,8 @@ decoration {
   box-shadow: 0 0 0 1px #20232b, 0 8px 8px 0 rgba(0, 0, 0, 0.35);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.9), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.35); }
+    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.9), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.35);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/gtk-solid-darker.css

@@ -895,6 +895,7 @@ headerbar,
     border-color: #262a33; }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(207, 218, 231, 0.5);
     background-color: #323843; }
     .csd headerbar:backdrop,
@@ -2857,7 +2858,8 @@ decoration {
   box-shadow: 0 0 0 1px #20232b, 0 8px 8px 0 rgba(0, 0, 0, 0.2);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.9), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2); }
+    box-shadow: 0 0 0 1px rgba(32, 35, 43, 0.9), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/gtk-solid.css

@@ -895,6 +895,7 @@ headerbar,
     border-color: #d4d5db; }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(82, 93, 118, 0.5);
     background-color: #eff0f2; }
     .csd headerbar:backdrop,
@@ -2861,7 +2862,8 @@ decoration {
   box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 rgba(0, 0, 0, 0.2);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2); }
+    box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/gtk.css

@@ -895,6 +895,7 @@ headerbar,
     border-color: rgba(212, 213, 219, 0.95); }
   headerbar:backdrop,
   .titlebar:backdrop:not(headerbar) {
+    transition: 200ms ease-out;
     color: rgba(82, 93, 118, 0.5);
     background-color: #eff0f2; }
     .csd headerbar:backdrop,
@@ -2861,7 +2862,8 @@ decoration {
   box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 rgba(0, 0, 0, 0.2);
   margin: 10px; }
   decoration:backdrop {
-    box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2); }
+    box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.1), 0 8px 8px 0 transparent, 0 5px 5px 0 rgba(0, 0, 0, 0.2);
+    transition: 200ms ease-out; }
   .fullscreen decoration,
   .tiled decoration {
     border-radius: 0; }

common/gtk-3.0/3.20/sass/_common.scss

@@ -3,6 +3,7 @@
 }
 
 $ease-out-quad: cubic-bezier(0.25, 0.46, 0.45, 0.94);
+$backdrop_transition: 200ms ease-out;
 $asset_suffix: if($variant=='dark', '-dark', ''); // use dark assets in dark variant
 $darker_asset_suffix: if($darker=='true', '-dark', $asset_suffix);
 
@@ -962,6 +963,8 @@ headerbar,
   }
 
   &:backdrop {
+    transition: $backdrop_transition;
+
     color: transparentize($header_fg, 0.3);
     background-color: opacify($header_bg_backdrop, 1);
 
@@ -3007,6 +3010,8 @@ decoration {
     box-shadow: 0 0 0 1px if($darker=='true' or $variant == 'dark', transparentize(darken($header_bg, 7%), 0.1), $_wm_border),
                 0 8px 8px 0 transparent,
                 0 5px 5px 0 if($variant == 'light', opacify($_wm_border, 0.1), transparentize($_wm_border, 0.2));
+
+    transition: $backdrop_transition;
   }
   .fullscreen &,
   .tiled & {