update

src/gnome-shell/sass/widgets/_dialogs.scss

@@ -1,14 +1,14 @@
 /* Modal Dialogs */
 
-$mdborder_color: if($variant == 'light', transparent, rgba($white, 0.15));
+$mdborder_color: if($variant == 'light', rgba(black, 0.15), rgba(black, 0.75));
 
 .headline { @include font(title); }
 
 .modal-dialog {
-  border-radius: $buttons_radius * 2;
+  border-radius: 20px + $container_padding;
   color: $fg_color;
   background-color: $base_color;
-  border: 0 solid $mdborder_color;
+  border: 0 none $mdborder_color;
   box-shadow: inset 0 1px rgba(white, 0.1);
 
   .modal-dialog-content-box {

src/gnome-shell/shell-40-0/gnome-shell-compact.css

@@ -1302,10 +1302,10 @@ StScrollBar StButton#vhandle:active, StScrollBar StButton#hhandle:active {
 }
 
 .modal-dialog {
-  border-radius: 16px;
+  border-radius: 24px;
   color: rgba(0, 0, 0, 0.87);
   background-color: #ffffff;
-  border: 0 solid transparent;
+  border: 0 none rgba(0, 0, 0, 0.15);
   box-shadow: inset 0 1px rgba(255, 255, 255, 0.1);
 }
 

src/gnome-shell/shell-40-0/gnome-shell-dark-compact.css

@@ -1302,10 +1302,10 @@ StScrollBar StButton#vhandle:active, StScrollBar StButton#hhandle:active {
 }
 
 .modal-dialog {
-  border-radius: 16px;
+  border-radius: 24px;
   color: rgba(255, 255, 255, 0.9);
   background-color: #202020;
-  border: 0 solid rgba(255, 255, 255, 0.15);
+  border: 0 none rgba(0, 0, 0, 0.75);
   box-shadow: inset 0 1px rgba(255, 255, 255, 0.1);
 }
 

src/gnome-shell/shell-40-0/gnome-shell-dark.css

@@ -1302,10 +1302,10 @@ StScrollBar StButton#vhandle:active, StScrollBar StButton#hhandle:active {
 }
 
 .modal-dialog {
-  border-radius: 16px;
+  border-radius: 26px;
   color: rgba(255, 255, 255, 0.9);
   background-color: #202020;
-  border: 0 solid rgba(255, 255, 255, 0.15);
+  border: 0 none rgba(0, 0, 0, 0.75);
   box-shadow: inset 0 1px rgba(255, 255, 255, 0.1);
 }
 

src/gnome-shell/shell-40-0/gnome-shell.css

@@ -1302,10 +1302,10 @@ StScrollBar StButton#vhandle:active, StScrollBar StButton#hhandle:active {
 }
 
 .modal-dialog {
-  border-radius: 16px;
+  border-radius: 26px;
   color: rgba(0, 0, 0, 0.87);
   background-color: #ffffff;
-  border: 0 solid transparent;
+  border: 0 none rgba(0, 0, 0, 0.15);
   box-shadow: inset 0 1px rgba(255, 255, 255, 0.1);
 }