Adwaita: set the right background color for message dialogs

gtk/theme/Adwaita/_common.scss

@@ -2779,6 +2779,12 @@ GtkCalendar {
 
 .message-dialog { // Message Dialog styling
   -GtkDialog-button-spacing: 0;
+
+  @if $variant==light {
+    // the bright variant as a slightly darker bg which in this case clashes with the window decoration, overriding
+    &.background { background-color: $bg_color; }
+  }
+
   .titlebar {
     border-style: none;
     box-shadow: inset 0 1px $borders_edge;

gtk/theme/Adwaita/gtk-contained.css

@@ -3972,6 +3972,8 @@ GtkCalendar {
 
 .message-dialog {
   -GtkDialog-button-spacing: 0; }
+  .message-dialog.background {
+    background-color: #ededed; }
   .message-dialog .titlebar {
     border-style: none;
     box-shadow: inset 0 1px white; }